Force equalizing filament clamp

ABSTRACT

A filament holding fixture having a closed position for applying substantially equal pressure to a plurality of points around the circumference of a filament. The holding fixture comprises a first jaw having a planar surface and an open-ended channel formed in the first jaw opening to the planar surface, and a second jaw having a structured surface including an open-ended groove of substantially rectangular cross section. Both the channel and the groove are sized for even application of pressure to a filament placed between them when there is axial alignment of the channel with the groove. An angular compensator may be included to facilitate positioning of the groove for even application of pressure to a filament when the planar surface lies adjacent to the structured surface, and the filament holding fixture is in the closed position.

FIELD OF THE INVENTION

The invention relates to a process and equipment for convenientlyhandling a filament in the form of an optical fiber during multipleprocessing operations that may be at least partially automated. Moreparticularly the invention relates to compact handling of optical fibersduring manufacturing operations to include Bragg gratings in at least aportion of their length via a series of manufacturing operationsincluding mechanical stripping, acid stripping, Bragg grating writing,and optical fiber recoating and testing.

BACKGROUND OF THE INVENTION

Glass has been used for centuries as a material of choice in a varietyof scientific and domestic applications. From the early use of prismaticglass for separating light into its component colors, glass has beenwidely used in optical devices that control or adjust the properties oflight beams. A recent and rapidly expanding application of the lightmodifying properties of glass structures involves the drawing of finefilaments of highly purified glass, more commonly referred to as opticalfibers, that direct light signals between light transmitting andreceiving locations.

During the late 1970s utilities began using optical fiber installationsfor internal communication, and by the early 1980s, a number of smalloptical fiber networks were installed. The use of such networks has beengrowing ever since to replace existing coaxial cable systems. Advantagesprovided by optical fiber communications networks include lower cost,the use of fewer signal repeaters to correct for signal distortion, anda higher signal carrying capacity than coaxial cable networks.

The capacity of fiber optic systems continues to increase. In 1980, thefirst systems could transmit 45 megabits per second. Current systemstransmit up to 5 gigabits per second. So extensive is the modern use ofoptical fiber networks that fiber optic networks have essentiallyreplaced all transcontinental copper cable networks and entirely newnetworks are being created continually. One prediction claims that everycontinent in the World will become part of a global fiber optic network.

A fiber optic system includes three main parts of transmit circuitry andlight source, light detector and receiver circuitry, and fiber. Thetransmit circuitry converts electronic signals to modulate a lightsource that generates light signals for transmission. Connection fromthe light source to a length of optical fiber facilitates transmissionof light signals for distances covered by the optical fiber. Attachmentof light detector and receiver circuitry at the terminal end of a fiberproduces a communication link The use of multiple communication linksprovides extended networks of transmitters and receivers.

Interconnection of fiber optic networks requires high precision devicesin the form of optical connectors that join optical fibers to peripheralequipment and other optical fibers while maintaining adequate signalstrength. In operation an optical connector centers the small fiber sothat the light gathering core lies directly over and in alignment with alight transmitting source or another fiber. Sections of optical fibermay also be spliced together using mechanical splicing or fusionsplicing techniques.

Special features may be built into selected, relatively short lengths ofoptical fibers to be spliced into fiber optic networks. A fiber Bragggrating represents a light-modifying feature that may be introduced orwritten into an optical fiber by simple exposure to ultraviolet light.The ability to write such gratings leads to a variety of devices. Forexample, Bragg gratings may be applied in telecommunications systems tocontrol the wavelength of laser light, to introduce dispersioncompensation, and, in the form of long period gratings, to modify thegain of optical fiber amplifiers. Fiber optic applications of fiberBragg gratings, outside of telecommunications, include spectroscopy andremote sensing.

The process of introducing special features such as Bragg gratings intoan optical fiber may include a number of steps requiring handling ofrelatively short lengths of optical fiber during a series ofmanufacturing operations. An optical fiber typically requires removal ofprotective coatings before changing the physical characteristics of thefiber to include a Bragg grating. After writing a Bragg grating, thefiber may be annealed and recoated.

Little has been revealed about the automation of processes to alter thecharacteristics of a fiber to provide it with a refractive indexgrating. Some evidence exists of individual processing steps but not ofa type that may be readily incorporated in an automated sequence. Fiberloading for example is described in U.S. Pat. No. 5,988,556. This patentrefers to automated winding of a continuous length of fiber from a fibersupply onto first and second sections of a shipping spool. The windercomprises a first device that collects a first portion of a continuouslength of fiber and winds it onto the first section of the spool, and asecond device for winding a second portion of the continuous fiber ontothe second section of the spool. There is no evidence to show that thespooled fiber has a use other than as a shipping package. U.S. Pat. No.6,027,062 describes an automated winder including fiber supply andcollecting devices that move a fiber to a threading device thatautomatically threads the fiber onto a spool. This is similar to thegoal of U.S. Pat. No. 4,511,095 to form of a coil of fiber wound onto abobbin or similar structure.

The use of stackable cassettes for handling and organizing opticalfibers is well known, particularly for storage of lengths of splicedfibers. Cassettes typically comprise shallow dish-shaped holders andenclosures for containment of loosely coiled optical fibers. Looseoptical fiber coils do not have the same compact structure as spooledoptical fibers. An intermediate form of coiled fiber, described in U.S.Pat. No. 5,894,540, may be produced using an assembly for holding alength of filamentary material in a wrapped configuration with a minimumbend radius. The filament or fiber may be wrapped around spools attachedto a support plate. Adjustment of the spacing between spools removesslack from the fiber wrapped around them. Fiber cassettes and relatedfiber holding assemblies place loose fiber in a tidy condition forstorage, usually following interconnection of lengths of optical fiber.U.S. Pat. No. 6,088,503 confirms the use of optical fiber cassettes asholders of optical fibers before, during and after splicing. The patentdescribes a clamping tool designed to align and hold a pair of fiberends in preparation for optical fiber splicing.

Cassettes and related fiber organizing assemblies provide tidy storagefor optical fibers around connected and spliced sections of opticalfiber. There appears to be no evidence of such storage containers usedfor processing organized lengths of optical fiber during the manufactureof optical fiber devices. One manufacturing process requires the removalof protective buffers and coatings to reveal the bare surface of anoptical fiber. Several processes are known for removing protectivelayers, such as buffers and coatings, from the surface of opticalfibers. They include mechanical stripping, chemical stripping andthermal stripping.

Mechanical stripping of optical fibers and related coated filamentsrequires careful positioning of sharp tempered metal blades to expose abare surface portion of a fiber without cutting or scratching orotherwise physically damaging the fiber surface. Known methods ofmechanical stripping relate to cutting blade design and how a coatingmay be removed from the surface of a fiber. The predominant use ofmechanical stripping involves the removal of protective layers from theends of optical fibers, insulated wires and related filaments, prior tojoining the filament ends together. U.S. Pat. No. 4,434,554 describes anoptical fiber stripping device including a flat base having a number offiber receiving channels of suitable depth to ensure only removal of abuffer coating from each fiber, when a blade penetrates the coating. Theblade moves parallel to the axis of a fiber or group of fibers using aparing action to remove protective material. Channel size, based uponfiber diameter determines the selection of a flat base to provide adevice that strips a fiber end without damaging the fiber itself.

One way to avoid damage to the bare surface of an optical fiber requiresthe use of blades designed to penetrate the protective buffer or fibercoating without reaching the fiber surface. Suitable blades have asubstantially semicircular sharpened edge of a radius slightly largerthan the radius of the bare optical fiber. Two opposing blades,penetrating the protective layer around the fiber, interfere with eachother before the cutting edges reach the fiber surface. Afterpenetrating a protective layer, close to the end of a fiber, movement ofthe blades parallel to the fiber axis displaces a section of the layerto provide a bare fiber end untouched by the blades. United StatesPatents, including U.S. Pat. Nos. 4,630,406, 5,269,206, 5,481,638, and5,684,910, describe the manufacture and design of blades for cuttinginsulation from e.g. insulated electrical wires and optical fibers.Successful mechanical stripping using such blades may require additionaltreatments, including softening the protective layer as in U.S. Pat. No.5,481,638 requiring a chemical filled chamber first to soften anencapsulating layer then to clean plastic material from the blades afterstripping. U.S. Pat. No.5,684,910 teaches an optical fiber havingimproved mechanical strippability. The improvement includes the use of afrangible boundary layer between a fiber coating and a buffer tofacilitate separation from the bare fiber. Previous teachings includeinitial blade movement perpendicular to a filament axis, to penetrate acoating, followed by movement parallel to the filament axis to exposebare filament ends by displacement of protective layers.

Chemical stripping may be used as an alternative to mechanical strippingfor preparing bare fiber ends. U.S. Pat. Nos. 4,865,411 and 4,976,596deal with controlled removal of coating, by gradual withdrawal of acoated fiber from a chemical bath, to produce a contoured shallow taperadjacent to the bare glass fiber surface. A fixture, according to U.S.Pat. No. 5,451,294 provides support while dipping the end of a coatedoptical fiber into a chemical bath to dissolve coating from the end.Chemical stripping methods include common problems related to thehandling of chemicals especially, as in this case, when the chemicalstrippers involve corrosive liquids.

Hot gas stripping may be used instead of mechanical or chemicalstripping. One example of this process, described in U.S. Pat. No.6,123,801, uses a hot inert gas to melt buffer coating and blow it fromthe surface of an optical fiber. The process requires high pressure gasstreams and temperatures in the region of 800° C. to strip coating fromthe fiber. U.S. Pat. No. 5,939,136 describes a process for preparingoptical fiber devices including thermal removal of a coating from anoptical fiber, preferably performed using a heated gaseous stream.

A reason for removing protective buffers and related coatings fromoptical fibers is the need to change the characteristics of the fibersuch as by writing of a refractive index grating, also known as a Bragggrating, in the core of an optical fiber. Refractive index changes occurduring exposure of a bare fiber to radiation from an ultraviolet laseror similar exposure device. The majority of protective coatings foroptical fibers absorb the fiber modifying radiation. This explains theneed to remove the coatings before writing a refractive index grating.

Without further processing, an optical fiber including a refractiveindex grating also has a bare portion that requires application ofprotective coatings before use in an optical fiber device. The widelyaccepted method for recoating bare sections of optical fibers involvesspecial coating molds. Methods similar to those used to coat drawnfibers, during their manufacture have also been described.

A recoating mold, described in U.S. Pat. No. 4,410,561, provides acoated optical fiber using a split mold die structure. The size anddesign of a cavity formed by the closed mold provides space that becomesfilled during injection of curable, protective, fluid recoatingcompositions. It is desirable to avoid entrapment of air inside the moldsince this could lead to a defective recoated fiber section. Completefilling of a mold cavity may involve intentional application ofpressure. U.S. Pat. No. 5,022,735 uses a screw type plunger topressurize recoating fluid injected into a conventional recoating mold.Some recoating molds include curing means to provide finished recoatedsections of optical fibers. U.S. Pat. No. 4,662,307, for example, uses asplit mold including an injection port and UV light port through whichlight passes to cure recoating compositions. The curing process requiresmultiple light sources.

Application of coatings to an optical fiber drawn from a pre-formtypically places the emerging fiber in a vertical orientation. As ittravels downward, the fiber may pass through a reservoir of coatingfluid before exiting through an orifice sized to the desired externaldiameter of the coated fiber. It is possible to apply such a process torecoating of bare sections of optical fiber including a Bragg grating,as taught in U.S. Pat. No. 6,069,988. Upon exit from the orifice, thefiber moves past a source of curing radiation. The curing radiationdiffers from the radiation used for writing the Bragg grating so as notto destroy or change the characteristics of the grating.

There is evidence in Japanese Patents JP 60-122754 and JP 61-40846 forspraying protective plastic coatings on optical fibers exiting a drawtower. Coverage of the full circumference of the optical fiber requiresthe uses of either multiple spray heads or special spray containmentshrouds. The use of multiple spray heads deposits only a fraction of thespray on the surface of the drawn fiber while the use of special shroudsinvolves complicated threading of a fiber.

Each point in the processes, of fiber stripping, modifying andrecoating, requires care to prevent damaging the fragile optical fiber.Damage to optical fibers may occur by physical contact or exposure totensile, torsional, twisting, and bending stresses. Excessive bendingcan change the optical characteristics of a fiber. Failure to meetrequired optical characteristics leads to rejection of an optical deviceand increases the expense of device manufacture. A need exists forimproved means for handling optical fibers for post draw processing, toreduce incidence of damage thereby reducing the cost and increasing theyield of optical fiber devices.

SUMMARY OF THE INVENTION

The present invention satisfies the need for effective and compacthandling of filamentary materials during manufacturing operationsincluding process steps that produce structural and related changes inthe filament. When applied to optical fibers, an article, also describedherein as a filament organizer, provides compact containment of anoptical fiber. The filament organizer allows relatively precisepositioning of at least a portion of an optical fiber to facilitateprocessing of optical fibers related to optical couplers, fused couplersand tapered fiber devices and the like. Optical fiber modification mayalso refer to actions taken to change the inherent characteristics of anoptical fiber or to incorporate an optical fiber into a functionalassembly. The inherent characteristics of an optical fiber change withadjustment of its refractive index properties, as in the formation of avariety of fiber Bragg gratings. Incorporation of an optical fiber intoa functional assembly provides useful devices such as temperaturecompensated fiber Bragg gratings. Refractive index changes andfunctional assembly production, according to the present invention, usea filament organizer that distributes an optical fiber between alockable spool and a rotary spool to expose a central portion of a fiberto be modified.

A computer controlled, or otherwise programmed, fiber dispenser may beused to load a prescribed amount of a substantially twist-free opticalfiber between a pair of spools mounted on a common axis. After fiberloading the spools are separated, with fiber extending between them, andmounted to a filament organizer for fiber storage and furtherprocessing. Use of computer controlled dispensing, combined with afilament organizer, allows accurate consistent loading and organizationof a selected length of optical fiber within the boundaries of thefilament organizer. Control of the loading process allows the productionof numerous holders containing approximately equal lengths of fiber,organized in similar fashion. After successful loading of an opticalfiber, a filament organizer provides a convenient article for handlingthe fiber through process operations required for the manufacture ofoptical fiber devices. Preferably the filament organizer includes meansfor applying a tension force between about 50 g to about 100 g to thefilament held therein.

A variety of devices use optical fibers that have been structurallymodified to include in-line optical waveguide refractive index gratingsin at least a portion of their length. Physical property variation ofgratings allows them to be tailored for specific applications. In oneembodiment, the present invention provides a fiber Bragg gratingobtained via a series of manufacturing operations including mechanicalstripping of an optical fiber, acid stripping, pigtailing, optical fiberBragg grating writing, annealing and optical measurement followed byrecoating and testing. The final step of testing, including fiber prooftesting, confirms attainment of performance requirements desired of anoptical fiber Bragg grating.

Each operation or step of the manufacturing process requires attachmentof one or more filament organizers to one or more filament processors orapparatus designed specifically to accomplish a designated step. Thisrequires that the size and shape of a filament organizer include aspectsof design allowing convenient connection with several filamentprocessors. As well as making suitable connection with several types offilament processors, an important requirement of a filament organizer iscontainment of a prescribed length of filament that may be up to severalmeters in length. Preferably, in the case of an optical fiber, afilament organizer holds most of the length of a filament on a pair ofspools leaving a portion of filament available for processing. A spoolholds two sections of optical fiber wound in the same direction onseparate sides of a divided spool core. One section of fiber extendsbetween a pair of spools while the other section of optical fiberprovides a pigtail portion that may be readily unwound from each spool.There is a pigtail section at each end of a continuous length of opticalfiber.

After winding a continuous length of optical fiber between a pair ofspools and positioning the spools on a support board, fiber handling mayproceed with reduced expectation of damage to the fiber. Also the use ofa filament organizer allows ready access to a portion of fiber. Readyaccess to this portion of fiber allows it to be modified initially byremoval of protective coatings from its surface and thereaftersubjecting it to operations that change its physical and opticalproperties, as in the writing of a fiber Bragg grating into a baredportion of optical fiber. A filament organizer allows reproduciblepositioning of that portion of an optical fiber that will be modified.Reproducible positioning leads to predictable results of filament oroptical fiber modification by operations that may be conducted using aprocess where at least several of the steps may be automated.

As indicated previously, a filament organizer provides a portion offilament or optical fiber suitably positioned for processing. Formationof an optical fiber Bragg grating according to the present inventionrequires that any polymeric protective coating, also referred to hereinas a buffer coating, should be removed prior to the writing of the fiberBragg grating. The coating may be removed using liquid or mechanical orthermal stripping.

An optical fiber covered with a single polymeric layer, referred toherein as a primary buffer, may require only liquid stripping usingconcentrated acid to remove the buffer. Removal of multiple protectivecoatings, including primary and secondary buffers according to thepresent invention, preferably uses a combination of mechanical strippingfollowed by acid stripping. Acid stripping herein refers to dissolvingresidual coating material in an acid medium with displacement of theacid using a water rinse and solvent wash applied to at least a portionof the fiber. Initial displacement of coating requires speciallydesigned mechanical stripping equipment that cooperates with a filamentorganizer for precise positioning of the portion of an optical fiberfrom which protective coating will be displaced. Mechanical strippingequipment may be designed for conveniently processing one filamentorganizer or several combined in a single stacked configuration. Thisresults in treatment of one or more fibers at a time depending on thenumber of filament organizers. Coating displacement, via mechanicalstripping, creates gaps to the bare fiber through which acid maysubsequently penetrate to more rapidly dissolve coating from the fiberportion.

Removal of coating by acid stripping preferably requires an apparatusthat forms a loop of filament for each filament organizer included in astacked configuration. The apparatus is constructed for formation ofindividual filament organizer loops having approximately the same size.The plane of each loop parallels that of its nearest neighbors. Acidstripping of one or more fiber loops occurs by immersing the arcuateportion of a loop into an acid bath. The depth of immersion of each loopinto the acid bath controls the length of protective coating removedfrom a fiber to provide an optical fiber having a bare portion strippedto the silica surface of the fiber. Acid stripping provides a bare fibersurface that is substantially free from contaminants.

After all of the fibers in a stacked configuration have beenmechanically stripped and acid stripped, the pigtail ends of each fiberare manually unwound and organized into groups using pigtail connectors.Pigtail ends trail about one meter from each end of a filamentorganizer.

As a further refinement, a filament organizer according to the presentinvention may include a conventional optical fiber connector forterminating optical fiber ends on the surface, and within the boundariesof the filament organizer. Optical connector termination of fibersreduces the length of pigtail portions of an optical fiber while stillproviding convenient points of attachment to external optical fiberdevices. Compact fiber organization of this type distributes the lengthof an optical fiber on the surface of a filament organizer without anypart of the fiber hanging over the edges of the organizer. Any of avariety of optical fiber device interconnects may be used to reduce theoverall length of an optical fiber by shortening the pigtail ends.Reduction in the overall length of an optical fiber translates into costsavings associated with each filament organizer equipped with pigtail tooptical fiber connector termination.

Following organization by grouping of pigtail ends each filamentorganizer in a stacked configuration provides a clean, dry, bare fiberportion ready for positioning in a fiber Bragg grating writingapparatus. After release of tension from a filament held by a filamentorganizer, the Bragg grating writing apparatus applies a selectedtension to the portion of an optical fiber before it is modified toproduce a Bragg grating. Production of multiple optical fiber Bragggratings, having a substantially identical wavelength response, requiresprecise alignment and application of the same amount of tension to eachoptical fiber portion loaded into the fiber Bragg grating writingapparatus. Precise alignment of an optical fiber portion with the Bragggrating writing apparatus relies on features built into a filamentorganizer and the grating writing apparatus respectively for consistentrelative positioning of one to the other. Consistent loading and fiberportion tensioning relies upon the use of a voice coil drive mechanismand air suspended bearings that facilitate accurate fine adjustmentessentially free from drag.

After placing an appropriate portion of an optical fiber under tensionin the fiber Bragg grating writing apparatus, the progress of Bragggrating writing may be monitored by observing a display of thewavelength envelope produced by the writing process. Signal informationproceeds from an optical fiber to suitable monitoring equipment throughconnections between the equipment and pigtail ends of a fiber. Thisprovides feedback of the quality of a grating at the time of writing andrepresents a convenient decision point for acceptance or rejection afiber Bragg grating as it is written.

Annealing of fibers takes place in a thermal annealing apparatus andfulfils several requirements upon completion of writing of fiber Bragggratings. This step of the process proceeds at a temperature ofapproximately 300° C. for a duration of more than about three minutes.The annealing process stabilizes the Bragg grating against wavelengthdrift for time periods exceeding about twenty to about twenty-fiveyears.

After annealing and optical confirmation that the grating centerwavelength meets requirements, the fibers and associated Bragg gratingsare ready for recoating before final testing. The recoating operationuses equipment designed for a filament organizer or preferably a stackedconfiguration of filament organizers according to the present invention.It is possible to use in-mold recoating, spray recoating or an extrusiondie coating process to recoat the previously stripped portion of eachoptical fiber. Injection die coating refers herein to conventionalin-mold die recoating. Spray recoating uses multiple passes of anoptical fiber between a spray head and a radiation curing source. Theextrusion recoating process uses a split die that may be positionedaround an optical fiber for application of a curable coating compositionaround the circumference of the fiber as the extrusion head traversesthe length of an uncoated fiber portion . Preferably the die headincludes a radiation source and the extruded coating cures by exposureto the radiation source. This allows application of recoating materialfollowed immediately by curing.

Application of recoating material to protect a Bragg grating formed inan optical fiber represents the final processing operation for producingfiber Bragg gratings that may be used in telecommunications and relatedapplications. A final check of the resulting product determines if itpasses tensile strength and visual inspection requirements. Aftersuccessfully meeting requirements, the spools holding a finished opticalfiber Bragg grating may be detached from the filament organizer and usedfor conveniently holding, packaging and transporting the final product.A convenient form of packaging for transportation requires transfer ofthe full continuous length of a fiber Bragg grating to one spool afterremoving it from the filament organizer. The design of a spool providesa protective cover for the fiber Bragg grating element followingtransfer of the full length of optical fiber to one spool.

More particularly, the present invention provides a method formanufacturing an optical fiber refractive index grating. A suitablemethod comprises the steps of providing a substantially twist-freelength of an optical fiber between a first spool and a second spool, forattachment of the first spool and the second spool to a support. Thesupport has a first surface opposite a second surface, to provide afilament organizer including the first spool as a lockable spool and thesecond spool as a rotary spool. The filament organizer further comprisesa tensioner coupled to the rotary spool to apply tension to at least acentral portion of the length of an optical fiber disposed between thelockable spool and the rotary spool. Further processing of a fiber undertension includes removing at least a buffer coating from the centralportion of an optical fiber before applying a controlled tension to thecentral portion of an optical fiber. A refractive index grating may thenbe written by changing the refractive index characteristics of thecentral portion during exposure of the central portion to aninterference pattern of high intensity actinic radiation, to produce therefractive index grating. After formation the grating may be annealedand the resulting fiber device proof tested to confirm desiredperformance properties.

The method described previously uses a filament organizer, comprising asupport having a first surface opposite a second surface and furtherincluding organizing mounts joined to said first surface and spacerblocks attached to said second surface. The filament organizer has alockable spool adjacent to the first surface of the support, a rotaryspool adjacent the first surface of the support, and a tensionerattached to the second surface of the support. The tensioner includes atension wire for attachment to the rotary spool to apply tension theretoto transmit tension to a filament disposed between the lockable spooland the rotary spool. A tension relief assembly allows selectivereduction of tension applied to a filament. The tension relief assemblyincludes the tension wire, providing connection between the tensionerand the rotary spool, a tension wire access, and at least one pulley foraligning the tension wire with the tension wire access. Other parts ofthe filament organizer include at least one mounting plate integrallyformed with the support and extending outwardly therefrom, and at leastone guide defining a filament path between the lockable spool and therotary spool. Further the guide is rotationally mounted on the mountingplate, adjacent to the first surface of the support, to provide spacingof the filament path from the support.

During refractive index grating manufacture a mechanical strippingapparatus displaces resin from a resin covered filament, in the form ofan optical fiber, by forming a removable sleeve portion between opposingfilament ends. The mechanical stripping apparatus comprises a base thathas a first clamp attached to the base to hold a filament at a firstlocation. A second clamp is attached to the base and has a separationfrom the first clamp and is in axial alignment therewith for holding afilament at a second location. The apparatus includes a first set ofcutting blades mounted on the base adjacent to the first clamp. Thefirst set of cutting blades includes a first upper blade and a firstlower blade. Each of the upper and lower blades includes an arcuatesharpened edge for cutting into resin around a resin covered filamentproximate to the first location. A second set of cutting blades ismounted on the base adjacent to the second clamp such that a distanceseparates the first set of cutting blades from the second set of cuttingblades. The distance between cutting blades is less than the separationbetween the clamps. The second set of cutting blades includes a secondupper blade and a second lower blade with each blade including anarcuate knife edge for cutting into resin around a resin coveredfilament proximate to the second location. A blade actuator secured tothe base, and coupled to the first set of cutting blades and the secondset of cutting blades, moves the first upper blade and the first lowerblade together. During this movement the sharpened edges penetrate resinaround a resin covered filament proximate to the first location. Theblade actuator also moves the second upper blade and the second lowerblade together for the knife edges to penetrate resin around a resincovered filament proximate to the second location. A biasing componentalso on the base moves the first set of cutting blades and the secondset of cutting blades towards each other during displacement of resinfrom a resin covered filament to form the removable sleeve portion.

The removable sleeve portion may be formed using a method for displacingresin from a resin covered optical fiber between opposing fiber ends.The method provides a mechanical stripping apparatus comprising a firstclamp for holding an optical fiber at a first location, a second clamphaving a separation from the first clamp and in axial alignmenttherewith for holding an optical fiber at a second location. A first setof cutting blades, of the mechanical stripping apparatus, is adjacent tothe first clamp for cutting into resin around a resin covered opticalfiber proximate to the first location. A second set of cutting blades isadjacent to the second clamp for cutting into resin around a resincovered optical fiber proximate to the second location. A distanceseparates the first set of cutting blades from the second set of cuttingblades. The distance is less than the separation between the first andsecond clamps. The first set of cutting blades and the second set ofcutting blades are adapted for movement towards each other duringremoval of resin from a resin covered optical fiber to form theremovable sleeve portion. Resin displacement further includes clampingan optical fiber in the first clamp and clamping the optical fiber inthe second clamp such that the optical fiber is under tension. Operatingthe first set of cutting blades and the second set of cutting blades,for cutting into the resin, produces the removable sleeve that has a gapat each end thereof. The gap at each end exposes a bare filament portionseparating the removable sleeve portion from a tapered transition formedin the resin during cutting of the resin as the first and second set ofcutting blades move towards each other.

In another aspect according to the present invention an apparatus may beused to form a loop in a section of a filament prior to chemicalstripping of resin from e.g. an optical fiber. The apparatus comprises acontainer including a front wall having a front guide slot formedtherein and a rear wall having a rear guide slot formed therein coplanarand parallel to the front guide slot. The container further includes afloor containing at least one slit formed between and parallel to thefront wall and the rear wall. A first filament gripper includes astationary elastomer roller and a positionable cylinder holding afilament therebetween, at a first location thereof. The stationaryelastomer roller is rotatably mounted from the front wall to the rearwall, so that the positionable cylinder is mounted, adjacent to thestationary elastomer roller, between the front guide slot and the rearguide slot for repositioning therein. A second filament gripper includesa movable elastomer roller and a movable cylinder holding the filamenttherebetween, at a second location. The second filament gripper has aseparation from the first filament gripper and has substantially axialalignment therewith. The second filament gripper moves towards the firstfilament gripper to reduce the separation to bring the first locationcloser to the second location thereby producing a loop of filamentbetween the first filament gripper and the second filament gripper. Theloop of filament extends through a slit to below said floor of thecontainer where it may be introduced into a reservoir having a solventtherein to surround at least a portion of the loop of filament todissolve resin from the portion of the loop. A loop forming containeraccording to the present invention may be sized to accommodate one ormore filament organizers having a filament between a lockable spool anda rotary spool. Steps for forming one or more filament loops using aloop forming container may be included in a process for chemicallystripping resin from a resin coated filament, preferably as an opticalfiber.

Processing of a filament according to the present invention requires theuse of a filament holding fixture comprising a gripper having an openposition and a closed position. The gripper further comprises a lowerjaw mount, and a lower jaw connected to the lower jaw mount, the lowerjaw having a planar surface and an open-ended, V-shaped channel formedtherein opening to the planar surface to receive at least a portion of afilament. The filament holding fixture also has an upper jaw mount, andan upper jaw assembly. The upper jaw assembly comprises a support flangeattached to the upper jaw mount. The support flange includes a supportsurface, having a substantially conical recessed portion. A fiber clasp,included in the upper jaw assembly, has a contact face opposite astructured surface. The structured surface includes an open-ended grooveof substantially rectangular cross section. There is a substantiallyconical depressed portion formed in the contact face of the fiber clasp.The open ended groove and the V-shaped channel are in longitudinalalignment to contact at least a portion of a filament when the gripperis in the closed position. A plurality of spring connectors hold thefiber clasp to the support flange. Also, an angular compensator isconfined between the recessed portion of the support surface and thedepressed portion of the contact face by force produced by the pluralityof spring connectors. The angular compensator maintains separation ofthe support flange from the fiber clasp to allow them to moveindependently. This leads to fine adjustment of the fiber clasp forapplying substantially equal force at points of contact of theopen-ended groove and the V-shaped channel with a filament, preferablyan optical fiber, held therebetween following movement of the gripperfrom the open to the closed position.

The present invention further provides a filament tensioning apparatusfor releasably securing a filament under tension. The tensioningapparatus comprises a tensioning holder and a pair of grippers. Thetensioning holder includes at least one support bar, and a firstcarriage movably mounted at a first location on a support bar. The firstcarriage includes an upper surface having a first clamp and a voice coilmounted thereon for movement relative to a support bar. A secondcarriage is movably mounted at a second location on a support bar suchthat a separation exists between the first location and the secondlocation. The second carriage includes an upper face having a secondclamp and a load cell mounted thereon for movement relative to a supportbar. The second clamp is in axial alignment with the first clamp tosecure a measured filament portion including a bare portion thereof,located inside a first boundary and a second boundary, between the firstclamp and the second clamp. A guide bar extends from the voice coil forcontact with the load cell to adjust the separation of the firstlocation from the second location, to change tension applied to themeasured filament portion, during activation of the voice coil. The pairof grippers of the tensioning apparatus is in axial alignment with thefist clamp and the second clamp, to substantially immobilize the bareportion of the measured filament portion. A filament tensioningapparatus according to the present invention may include a coupling forattaching a filament organizer to position a filament, preferably anoptical fiber, to be held between the first clamp and the second clamp.The filament organizer holds a filament between a lockable spool and arotary spool.

A resin covered filament having had resin removed therefrom may requirecoating by a method that uses a filament recoating apparatus accordingto the present invention. Such a filament recoating apparatus comprisesa frame for releasably securing a filament and a carriage mounted on theframe to oscillate between a first position and a second position. Therecoating apparatus has a first filament holding fixture mounted on thecarriage. A second filament holding fixture is also mounted on thecarriage in axial alignment with the first filament holding fixture. Thefixtures secure a measured filament portion including a bare portionthereof, located inside a first boundary and a second boundary, betweenthe first filament holding fixture and the second filament holdingfixture. At least one spray head is attached to the frame at the firstposition. A radiation source is attached to the frame at the secondposition. The measured filament portion moves between the spray head andthe radiation source, during oscillation of the carriage between thefirst position and the second position to place the bare portion toreceive a curable coating from the spray head. The spray head appliescurable coating from the first boundary to the second boundary. Curingof the curable coating occurs by exposure to radiation from theradiation source. Droplets of curable coating composition may bedeflected using a deflector, such as an air-knife, to selectively directcoating composition towards a plurality of bare filament portions offilaments, preferably optical fibers, grouped around a spray head.Different coating compositions may be applied to bare filament portionsto provide recoated filaments using a first composition and overcoatedfilaments by application of a second coating composition over the firstcoating composition. The resulting filaments include a multilayercoating.

An alternative filament recoating apparatus, according to the presentinvention, comprises a planar surface and an extrusion coating assemblyattached to the planar surface. The extrusion coating assembly comprisesa first filament holding clamp and a second filament holding clampopposite the first filament holding clamp. A measured filament portionincluding a bare portion thereof, located inside a first boundary and asecond boundary, lies between the first filament holding clamp and thesecond filament holding clamp. A coating head, includes a die platehaving formed therein an open ended channel including a wall having afluid entry and a gas port formed therein adjacent a radiation source.The coating head further includes a cover die plate having formedtherein an open ended elongate slot. The cover die plate has a hingedconnection to the die plate for rotation of the cover die plate betweenan open position and a closed position. In the closed position the coverdie plate lies adjacent to the die plate and the channel aligns with theelongate slot to form a tubular opening through the coating head toencircle a section of the bare portion. A linear transport mechanismadjacent to the coating head includes a guide rod and a carriageslidably mounted thereon for movement along the guide rod. A connectingrod from the carriage to the coating head provides linear displacementof the coating head during movement of the carriage to move the coatinghead from the first boundary to the second boundary. Curable fluid maybe extruded from the fluid entry while energy from the radiation sourcecures the curable fluid to recoat the bare portion of a filament.

A method for extrusion coating a filament comprises the steps ofproviding a filament organizer having an extended filament between afixed spool and a rotary spool to provide a measured filament portionand a bare filament portion of a filament, preferably an optical fiber.Recoating of the bare portion of a fiber follows attachment of thefilament organizer to an extrusion coating fixture comprising a guiderod, a carriage movably mounted on the guide rod. A coating die,including a coating head and a radiation source, is joined to thecarriage. The coating head has an opening for directing a curablecoating composition to the bare filament portion positioned in a channelformed in the coating die and extending therethrough. A curable coatingcomposition is applied to the bare filament portion to provide arecoated filament portion, followed by exposing the recoated filamentportion to the radiation source for radiation curing of the curablecoating composition applied to the bare portion.

Definitions

The terms “bare fiber,” or “bare fiber portion,” or “stripped fiber,” orphrases relating to such terms refer herein to the portion of an opticalfiber from which protective coating has been removed to expose thesilica surface of the fiber.

As used herein, the term “cladding” refers to the outer layer of anoptical fiber, as drawn.

The term “buffer” or “primary buffer” refers herein to a polymer orresin layer next to a bare fiber.

A “coating” or “secondary buffer” is used herein to describe a polymeror resin layer next to a buffer or primary buffer.

The term “resin” as used herein is a general term describing polymercoverings for filaments particularly optical fibers. Materials used forpreviously defined buffers and coatings fall within the general term ofresin.

The term “filament” herein refers to a fiber structure, preferably a“silica filament.” An optical fiber is a preferred form of filamentaccording to the present invention.

A “tapered transition” describes the preferably graduated conical shapeof the portion of buffer layers closest to a bare fiber portion aftersubjecting a coated optical fiber to mechanical stripping according tothe present invention.

The term “ribbonizing” refers to the formation of a single layer ofoptical fibers, side by side, as a flat ribbon-like structure thatfacilitates the joining of ends of multiple fibers for insertion in oneend of a fiber optic ribbon connector.

The term “angular compensator” or “ball joint leveler” as used hereinmeans a self adjusting coupling inserted between parts of at least onejaw of a gripper to achieve optimum positional relationship between thecontacting surface of the jaw and an object to apply even pressure overthe surface of the object.

The use of a “non-contact” method for recoating bare portions of opticalfibers means that no portion of the fiber touches any part of therecoating equipment. This is a benefit of suspending a fiber in afilament organizer that may be readily attached to the recoatingapparatus with precise fiber to spray head alignment.

A “split sizing die” is a multi-part fiber recoating head that opens toreceive an optical fiber, closes to extrude curable recoating materialaround the surface of a length of fiber and re-opens to release thecoated fiber.

The term “shroud” refers to a shield over an ultrasonic spray head todirect a stream of inert gas to entrain and move a cloud of droplets ofrecoating composition towards a target surface, such as a bare portionof an optical fiber.

The present invention has been developed to provide a process andequipment for conveniently handling a filament in the form of an opticalfiber during multiple processing operations that may be at leastpartially automated as a further benefit to the user. These enhancementsand benefits are described in greater detail hereinbelow with respect tothe several aspects and alternative embodiments of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a perspective view of a filament organizer according to thepresent invention.

FIG. 2 provides detail portions of a filament organizer according to thepresent invention in exploded perspective view.

FIG. 3 shows a perspective view of an assembly for applying tension to afilament contained in a filament organizer according to the presentinvention.

FIG. 4 is a side elevation of a filament organizer.

FIG. 5 provides an perspective view of a plurality of filamentorganizers in a stacked configuration.

FIG. 6 is a cross sectional view through a dual coated optical fiber.

FIG. 7 shows a cross section of a dual coated optical fiber after acidstripping.

FIG. 8 provides a cross section of a dual coated optical fiber aftermechanical stripping to provide a tapered transition.

FIG. 9 shows a side elevation of an optical fiber after mechanicalstripping to produce a separated central buffer sleeve.

FIG. 10 is a cross sectional view showing a coated optical fiberpositioned for mechanical stripping.

FIG. 11 is a cross sectional view showing a coated optical fiber afterformation of a tapered transition

FIG. 12 shows a perspective view of a cutting blade according to thepresent invention for use during mechanical stripping of coating from anoptical fiber.

FIG. 13 provides a detail view of a cutting edge of a cutting bladeaccording to the present invention.

FIG. 14 is a detail view showing the relative positioning of a coatedoptical fiber and the cutting edge of a cutting blade according to thepresent invention.

FIG. 15 is a cross sectional view indicating the depth of cut of acutting edge, and positioning of an upper blade that has penetrated asecondary buffer around an optical fiber.

FIG. 16 provides a cross sectional view showing depth of cut of an upperblade and a lower blade during mechanical stripping of secondary bufferfrom an optical fiber.

FIG. 17 is a diagrammatic representation of a side elevation showing therelative positioning of a filament organizer and mechanical strippingapparatus according to the present invention.

FIG. 18 provides a cross sectional view of a coated optical fiber loopduring removal of coating by immersion in acid contained in an acidbath.

FIG. 19 is partial cross section showing the relative positioning of afilament organizer according to the present invention and anacid-containing bath before formation of an optical fiber loop.

FIG. 20 is partial cross section showing the relative positioning of afilament organizer according to the present invention including a loopsuspended from the filament organizer for immersion of an arcuateportion of the loop below the acid surface.

FIG. 21 is a perspective view including a cutaway section to showinternal detail of a loop former for a stacked configuration of filamentorganizers.

FIG. 22 provides a side elevational view of a filament organizeraccording to the present invention wherein pigtail portions of filamenthave been unwound from storage reels.

FIG. 23 is a diagrammatic side elevation of a filament organizeraccording to the present invention positioned in a voice-coil tensioningdevice during modification of a stripped portion of an optical fiber.

FIG. 24 provides a cross sectional view of a jaw used to preventmovement of a portion of an optical fiber while it undergoesmodification.

FIG. 25 is a detailed cross sectional view of the structure of the jawshown in FIG. 24.

FIG. 26 is a perspective view of a jaw assembly according to the presentinvention.

FIG. 27 is a cross section taken through line 27—27 of FIG. 26.

FIG. 28 is a diagrammatic side elevation of a filament organizeraccording to the present invention positioned in a spray recoatingapparatus.

FIG. 29 is a diagrammatic side elevation of a filament organizeraccording to the present invention positioned in a split die recoatingapparatus.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the Figures wherein like numbers refer to like partsthroughout the several views. FIG. 1 is a perspective view of a filamentorganizer 10 including a substantially planar support board 12 that hasopposing sides. The support board 12 has, on its first side 14, alsoreferred to herein as its upper side, points of attachment for alockable spool 16 and a rotary spool 18. Theses spools 16, 18 have alength of filament 20 wound around them and between them. The supportboard 12 may include mounting plates 22 for one or more guides 24 usedto establish a preferred position for a portion of a filament 20extending from the lockable spool 16 to the rotary spool 18. Attachmentof a guide 24 to a mounting plate 22 allows suitable movement of theguide in response to movement of the filament 20.

A support board 12, according to the present invention, optionallyincludes at least one opening 26 formed therein as a convenient locationfor holding a support board 12 by hand. An opening 26 preferablyoccupies a location sufficiently separated from an optical fiber 20 toprevent inadvertent touching of the surface, especially a bare surfaceof a filament 20, such as an optical fiber. The positioning of anopening 26 has suitable separation from a filament when located betweenthe lockable spool 16 and rotary spool 18 opposite an exposed portion ofa filament 20, as illustrated in FIG. 1.

FIG. 2 uses a partially exploded view to show detail of an embodiment ofa filament organizer 10 according to the present invention. This viewdepicts how a lockable spool 16, a rotary spool 18, and a pair of guides24 may be attached to a substantially planar support board 12. Assuggested earlier, a computer controlled fiber dispenser may be used toload a prescribed amount of optical fiber between a pair of spools 16,18. A continuous length of fiber preferably has three sections includinga main or central section, several meters in length separating oppositeend or pigtail sections, each approximately one meter long. Typicallythe central section has a length of from about one meter to about sixmeters. Each of the spools 16, 18 provides storage for an optical fiberpigtail. Filament turns representing pigtail sections are wound on aspool 16, 18 in the same direction as the filament turns of the centralsection. The spools 16, 18 each include a core divider (see FIG. 4) toseparate pigtails from the central section of an optical fiber 20 tofacilitate unwinding of pigtails during filament processing.

After fiber loading, the spools 16, 18 are separated and mounted to asupport board 12 for fiber storage with a length of filament stretchingbetween the spools 16, 18. One of the spools 16, 18 becomes a lockablespool 16 during sliding engagement between an axial channel 28 in thespool 16 and a post 30 secured to the support board 12. Each faceplate32 of the lockable spool 16 includes at least a pair of openings 34positioned on either side of the axial channel 28. The openings 34 alignfor engagement with a pair of pegs 36 rigidly connected to the supportboard 12. When the axial channel 28 and openings 34 seat over the post30 and pegs 36 the lockable spool 16 cannot rotate since the pegs 36restrict its movement. After mounting the lockable spool 16, asdescribed previously, a change in length of filament 20 between thelockable spool 16 and the rotary spool 18 requires adjustment of afilament 20 by rotating the rotary spool 18. Rotation of the rotaryspool 18 relies upon a bearing 38 held by friction in a hub orifice 40formed in the support board 12 of the filament organizer 10. The bearing38 facilitates rotation of a spool hub 42 on one side and tensioning hub44 on the other. The spool hub 42 has a spindle 46 and a pair of pins 48for alignment with an axial opening 50 and receiving holes 52 in bothfaceplates 54 of the rotary spool 18. Although held in fixedrelationship, the rotary spool 18 and spool hub 42 have rotationalfreedom provided by the bearing 38.

The embodiment of the present invention illustrated in FIG. 2 includes apair of guides 24 each taking the form of an idler wheel that rotatesaround an axle 56. One end of the axle 56 connects to the first side 14of the support board 12. Filament 20 un-spooling, by rotation of therotary spool 18, produces sufficient filament 20 between the lockablespool 16 and the rotary spool 18 so that the path of the filament 20from the lockable spool 16 to the rotary spool 18 passes around each ofthe idler wheels 24. The resulting assembly, including the filament 20,provides a preferred orientation, as shown in FIG. 1, when the filament20 is an optical fiber. This orientation places an optical fiber 20 in areadily accessible, spatially precise position for processing, duringthe manufacture of a fiber Bragg grating, for example.

Formation of an optical fiber Bragg grating frequently requiresapplication of tension to a filament 20, i.e. optical fiber, held by afilament organizer 10 according to the present invention. Reference toFIG. 3 indicates one means to apply tension to a filament 20 using atensioner 58 attached to the lower side 60 of a support board 12. Thetensioner 58 applies a constant force to a filament 20 or optical fiberto maintain the filament 20 under slight tension. Force from thetensioner 58 may be transmitted to the filament 20 through a series ofcomponents including a tension wire 62 connected between the tensioner58 and the tensioning hub 44. The tensioning hub 44 acts upon the spoolhub 42 because of a direct connection between the two. Movement of thespool hub 42 causes corresponding turning movement of the rotary spool18 which is centered on the spindle 46 and driven by the movement ofpins 48 attached to the spool hub 42. Movement of the rotary spool 18produces tension in the filament 20 or optical fiber proportional to theconstant force produced by the tensioner 58.

The tension in the tensioner 58 acts on the tension wire 62 with a forceof from 20 g to 200 g. Force distribution through the tension wire 62and the tensioning hub 44 leads to a resultant force of about 50 g toabout 100 g of tension in a filament 20 held in a filament organizer 10.A tensioning hub 44 is typically about half the diameter of a rotaryspool 18.

The tension wire 62 may pass unimpeded between the tensioner 58 and thetensioning hub 44. Preferably, however, the location of a section of thetension wire 62 allows access and positional adjustment of the tensionwire 62 to remove tension from the filament 20. Removal of the effect ofthe constant force tensioner 58 uses a force reduction assemblycomprising a pair of pulleys 64 on either side of a notch 66 formed inan edge of the support board 12. The tension wire 62, passing around thepulleys 64 and across the notch 66, may be grasped in the vicinity ofthe notch 66 and extended slightly parallel to the edge of the board 12,and toward the nearest guide 24. Movement of about 1.0 mm to about 2.0mm releases the tension force applied to a filament 20 by the tensioner58. Release of tension relaxes a filament 20, in the form of an opticalfiber, in preparation to re-tension the fiber. Relaxation andre-tensioning of an optical fiber 20 applies a known and repeatableamount of tension required by the Bragg grating writing process so thatresulting optical fiber Bragg gratings will exhibit substantially thesame wavelength response.

After installing a filament 20 under tension on a filament organizer 10,a length of filament several meters long may be conveniently carried, ina protected spooled condition, between handling stations, duringfilament processing. Earlier methods for filament processing required anoperator to hand-carry lengths of filament measuring in excess of sixmeters. Care was required to avoid contact of trailing filament with thefloor and other surfaces that could cause contamination and reduction inthe yield of manufactured filament devices. A filament organizer 10according to the present invention may be handheld using the opening 26in the support board 12. The position of the opening 26 minimizesundesirable inadvertent hand contact with any exposed portion of thefilament 20.

FIG. 4 shows a side elevation of a filament organizer 10 according tothe present invention to indicate the relative positioning of componentson the upper 14 and lower 60 side of a support board 12. The figureshows the tensioner 58, spacer blocks 70, tensioning hub 44, tensionwire 62 and a pulley 64 on the lower side of the support board 12. Theupper side of the support board 12 provides a surface for attachment oforganizing mounts 72, a lockable spool 16 and a rotary spool 18 having afilament or optical fiber 20 held under slight tension between them, asdescribed previously. The spools 16, 18 used to store a length ofoptical fiber 20 are divided spools since the lockable spool 16 includesa dividing wall 15 and the rotary spool has a partitioning wall 17. Partof an optical fiber 20 lying between the spools 16, 18 occupies a lowercore 19 of a spool 16, 18 below each of the dividing wall 15 and thepartitioning wall 17. This leaves the upper core portion 21 of eachspool, i.e. above the dividing wall 15 and the partitioning wall 17 toreceive a pigtail end of a continuous length of optical fiber 20.Separation of a central length from the ends or pigtails of an opticalfiber 20 using lower 19 and upper 21 core portions of storage spools 16,18 facilitates unwinding of only the pigtail portions of a fiber 20during fiber processing.

With suitable design, two or more filament organizers 10 may be combinedto form an assemblage of filament organizers 10. The term, stackedconfiguration 68, describes herein an assemblage of filament organizers10, as illustrated in FIG. 5. Design considerations include the placingof stabilizing spacer blocks 70 on the lower surface 60 of a supportboard 12 for mating registration with organizer mounts 72 positioned onthe upper surface 14 of a support board 12. The spacer blocks 70 mayhave insufficient height to hold a tensioner 58 clear of a planarsurface upon which a filament organizer 10 may be placed prior toforming a stacked configuration 68 thereon. This problem may be overcomeusing a suitably contoured spacer between the lower surface 60 of asupport board 12 and the planar surface. Correct stack 68 formationrequires addition of filament organizers 10 one on top of another withsuitable alignment of downward facing spacer blocks 70 and upward facingorganizer mounts 72 to produce a stable stacked configuration 68 bymating registration between filament organizers 10. The combined heightof spacer blocks 70 and organizer mounts 72 provides sufficient spacingbetween filament organizers 10. Alignment of spacer blocks 70 andorganizer mounts 72 produces a stacked configuration 68 neatlyorganizing a number of filaments 20, corresponding to the number offilament organizers 10, in a pre-determined relationship. Thisrelationship facilitates optimum orientation of a stacked configuration68 with filament processing equipment. The spacing between filaments 20in a stacked configuration 68 is from about 12.5 mm (0.5 inch) to about27.5 mm (1.2 inches) preferably about 18 mm (0.7 inch) to about 23 mm(0.9 inch).

Spacer blocks 70 and organizer mounts 72 may be viewed as primarycomponents for aligning one filament organizer 10 relative to itsnearest neighbor. Actual fiber 20 positioning and spacing depends uponthe location and relative height of the spacer blocks 70 and theorganizer mounts 72 on a support board 12. This means that the design ofa support board 12 determines the position of a length of filament 20 sothat it may be readily located in a stacked configuration 68. Inaddition to this, a filament 20 occupies a known position on a supportboard 12 with consistent spacing of the fiber 20 from other portions ofthe support board 12, such as the mounting plates 22 and upper side 14and lower side 60 of a support board 12. These features providereference points for uniting filament organizers 10, 68 to variouspieces of apparatus with one or more filaments 10 positioned ready forprocessing. This provides a filament organizer 10 and a stackedconfiguration 68 suitable for use in automated filament processingwithout operator intervention.

Means for handling multiple filament organizers 10 in a stackedconfiguration 68 includes installation of a stacking connector 74 thatoptionally includes a carrying handle 76. Preferably a stackingconnector 74 comprises one or more rods 78 inserted into through-holes80 (see FIG. 1) included in a support board 12. A rod 78 may include aflange as a support for the lowest filament organizer 10 in a stackedconfiguration 68. Alternatively, when a stacking connector 74 comprisesadditional rods 78 threaded through multiple filament organizers 10, abracket may be used to connect the ends of rods 78 adjacent to the lowerface 60 of the lowest filament organizer 10 in the stack 68. A carryinghandle 76 may be attached to a stacking connector 74 by any of a varietyof joining methods. Rods 78 protruding from a stacked configuration 68may include threaded end portions that may be received in tubularopenings (not visible) in a carrying handle 76. A threaded nut orsimilar retainer 82 may secure the handle 76 to the stacking connector74. This provides a stable stacked configuration 68 of multiple filamentorganizers 10 that may be carried with ease between stations forprocessing a plurality of filaments 20 in a single batch or automatedoperation.

A filament 20 in the form of an optical fiber, once installed in afilament organizer 10, requires a series of steps to produce refractiveindex modifying features in a selected portion of the optical fiber 20.Optical fiber manufacture, using a draw tower, typically includes theapplication of protective coatings over the length of the fiber.Identification of protective coatings for optical fibers uses a varietyof terms including buffer and coating. The term buffer usuallyidentifies a material coated directly on a bare optical fiber. A coatingusually designates a protective material coated over a buffer layer.

FIG. 6 is a cross sectional view of an optical fiber showing layers ofprotective coatings. As used herein the term primary buffer refers to abuffer coating 100 and the term secondary buffer 102 refers to a coatingapplied to the primary buffer 100. Optical fiber Bragg gratingmanufacture requires the removal of both the primary buffer 100 and thesecondary buffer 102 from a central portion of an optical fiber 20stored on a filament organizer 10. One method for stripping a buffercoat requires dipping the fiber in a hot concentrated sulfuric acidbath. Preferably the sulfuric acid concentration is at least 95% beforeheating and stripping the buffer coating 100, 102 from an optical fiber.Acid stripping occurs at a temperature above about 150° C. Damage to aglass core 106 is less likely to occur with acid stripping than withother methods used to remove buffer coats 100, 102, since glass isresistant to acid.

A hot acid bath provides an effective medium for removing a singlebuffer coat, but some types of optical fiber have multiple coatings thatmay dissolve at different rates. These types of optical fiber mayinclude a relatively insoluble, hard secondary buffer coating 102 over asofter protective primary buffer coating 100, as illustrated in FIG. 6and FIG. 7. During hot acid stripping, the softer primary buffer coating100 may dissolve faster than the secondary buffer coating 102. Theprocess of dissolving polymer layers from an bare optical fiber 106 maybe accompanied by decomposition due to depolymerizing sulfonation causedby the attack of the concentrated sulfuric acid. Polymer decompositionproducts may impair the appearance and performance of a modified opticalfiber according to the present invention.

Placement of the primary buffer coating 100 under the secondary buffercoating 102 can result in preferential removal by acid of the primarybuffer coating 100. Preferential removal of the primary buffer coating100 produces an undercut 104 below the secondary buffer coating 102 thatcan collapse inward towards the bare optical fiber 106. As the secondarybuffer coating 102 collapses it can trap acid or air bubbles next to thebare optical fiber 106. Entrapment of material including acid, and otherliquids or gases, can produce conditions leading to premature failure ofan optical fiber 20 for intended applications.

A contributor to premature failure, as indicated previously, may be theexistence of decomposed polymer species after strong acid treatment.This situation may be avoided using an intermediate, mechanicalstripping method to provide a cut tapered transition section 108, asshown in FIG. 8, between the primary coating 100 and an bare opticalfiber 106. The tapered transition 108 prevents the collapse of asecondary buffer coating 102, as previously described. When mechanicalstripping, according to the present invention, precedes acid strippingthe formation of a tapered end 108 can present a preferred geometry atthe junction between the primary buffer coating 100 and the bare surfaceof an optical fiber 106.

FIG. 9 shows how an intermediate, central portion of an optical fiber 20may be stripped from two points along its length using a pair of cuttingblades. The intermediate portion to be stripped preferably resides in afilament organizer 10 according to the present invention. A mechanicaloptical fiber stripping apparatus accommodates a filament organizer 10,providing correct orientation for stripping buffer coatings 100,102 froman optical fiber 20. The apparatus controls blades (not shown) that cutinto the secondary buffer coating 102 to produce a separated buffersleeve 110 between a pair of tapered transitions 108 that define thelength of bare optical fiber when the primary 100 and secondary 102buffer coatings have been removed. Each end of the buffer sleeve 110includes a peeled-back collar 112 that provides a gap for access to thebare surface of the optical fiber 106.

FIG. 10 and FIG. 11 provide clarification of the basic components andsteps required for stripping a central portion of a coated optical fiber20. A first clamp 120 holds one end of a portion of a coated opticalfiber 20. The coated optical fiber 20 comprises a fiber core 106overcoated with one or more protective resin layers 100, 102. A secondclamp 122 holds the other end of the portion of the coated optical fiber20. Both clamps 120, 122 grip the outer surface of a relatively hardsecondary buffer coating 102. This prevents damage to the underlyingoptical fiber core 106. Preferably the clamps 120, 122 includefrictional gripping surfaces such as rubber or elastomer grippingsurfaces that resist fiber movement during mechanical stripping.

The immobilized coated optical fiber exists under slight tension,preferably of about 50 g. Typical separation between the first clamp 120and the second clamp 122 is from about 50.0 mm (2.0 inches) to about 100mm (4.0 inches) preferably 75.0 mm (3.0 inches) to about 90 mm (3.5inches). After limiting optical fiber 20 movement between a pair ofclamps 120, 122 at least one set of cutting blades 124 may be placedabutting the coated optical fiber 20 with the sharp edge of an uppercutting blade 126 resting against the surface of the coating 102surrounding the optical fiber 20. The desired position is shown by thelocation of a first set of cutting blades 124 relative to the clamped,coated optical fiber 20. A second set of cutting blades 130 is shown inFIG. 10 in a position, adopted by the cutting blades 130, afterpenetration of the secondary buffer 102 of an optical fiber 20. Each setof cutting blades includes an upper blade 126 and a lower blade 128. Thesharp edge of each cutting blade 126, 128 includes at least one notchhaving a radius in common with any primary buffer coat 100 applied to anbare optical fiber 106. During fiber stripping, the upper 126 and lower128 blades move inwards, as shown for the second set of cutting blades130, cutting through the secondary buffer 102 until they touch oneanother, before penetrating the primary buffer 100. The distance betweenthe first set of cutting blades 124 and the second set of cutting blades130, at this point, is typically between about 30 mm (1.2 inches) andabout 40 mm (1.5 inches). After cutting through an outer or secondarybuffer coating 102, application of suitable force moves the second setof cutting blades 130 closer to the first set of cutting blades 124 andparallel to the axis of the optical fiber 20. This transverse movementof the second set of blades 130 generates a stripping action thatresults in a gap 132 in the coating around the optical fiber core 106.The stripping action exposes a bare fiber portion 106 in the gap 132.One side of this gap 132 has a contoured, tapered transition 108, in theprimary buffer 100, from the bare optical fiber 106 to the secondarybuffer coating 102. The other side of the gap includes a compressed,peeled-back collar 112 of stripped coating 100, 102. When the cuttingand stripping operations have been completed at one end of the coatedoptical fiber portion, the opposite end of the fiber 20 may be strippedby initiation of cutting action of the first set of cutting blades 124.This produces a second gap 134 in the coating around the bare fiber 106,as illustrated in FIG. 9. The second gap 134 includes a similar taperedtransition 108 to that produced by the cutting action of the second setof cutting blades 130.

Application of acid stripping to a mechanically stripped fiber, as inFIG. 9, preferably exposes only the buffer sleeve 110 to acid attack. Aslong as the tapered ends 108, also referred to herein as taperedtransitions, remain out of the strong aqueous acid they remain free fromattack and chemically unchanged. In this condition the taperedtransitions 108 have a surface energy more compatible with recoatingcompositions. This allows the recoating compositions to readily wet thesurface of the tapered transitions 108 following modification of thecentral portion of a filament. Surface compatibility and ready wettingby recoating compositions produces defect free junctions betweenpreviously coated and recoated sections of optical fibers. The existenceof defects, e.g. air bubbles, in transition areas of an optical fibermay adversely affect the mechanical strength and light transmissioncharacteristics of an optical fiber device, rendering it unsuitable forits intended use.

FIG. 12 shows the design of blade components used for cutting thesecondary buffer coating 102 and displacing the primary buffer coating100 to cause separation of the primary buffer coating 100 from a bareoptical fiber 106. As illustrated in FIG. 12, a blade 140 providesdetail of features that may be included in both sets of cutting blades124, 130. The blade includes provision for stripping several opticalfibers 20 simultaneously. It will be appreciated that the same blade 140may be used to strip single or multiple fibers 20 depending on thenumber of filament organizers 10 inserted into the stripping apparatus(see FIG. 17).

A stripping blade 140 according to the present invention includes atleast one bevel 142 as a portion of the blade 140 that includes severalchannels 144 machined into its surface. The channels 144 open to an edge146 of a bevel 142 as sharpened notches 148 having approximatelycircular cross-section when viewed from the side opposite the bevel 142,as in FIG. 14. A detail view, shown in FIG. 13 provides clarification ofthe structure of the bevel 142 including the channels 144 machinedtherein. A coated optical fiber 20 is included in FIG. 14 to indicateits preferred position before penetration of the secondary buffer 102 bya sharpened notch 148 of a cutting blade 140. The knife-edge of asharpened notch 148 preferably reaches only towards the outer surface ofthe primary coating 100 of an optical fiber 20 without cutting into it.When used for stripping coating from an optical fiber 20, the notches148 cut a circular path around an optical fiber core 106 as shown inFIG. 15 and FIG. 16. In the illustration of FIG. 15 a sharpened notch148 appears as it would after an upper cutting blade 126 penetrates thesecondary buffer 102 of an optical fiber 20. This relates to theposition of the second set of cutting blades 130 as shown in FIG. 10.The relationship between the upper blade 126 and lower blade 128 of thesecond set of cutting blades 130 appears in the diagrammaticrepresentation shown in FIG. 16. The sharpened notches 148 of the upperblade 126 and lower blade 128 have penetrated the secondary buffercoating 102 without reaching the surface of the primary buffer coating100. Since the advancing edges 146 of the blades 126, 128 have madecontact there can be no further advancement of either blade 126, 128.

Stripping blades 126, 128 according to the present invention performbiaxial movement. Initial movement of a blade 126, towards a fiber core,produces a cut as a blade penetrates the secondary buffer coating 102 ofthe fiber 20. After traveling the thickness of the secondary buffercoating 102, the blade begins to move toward the center of the opticalfiber 20, parallel to its axis. This movement disrupts the coating 100,102, producing a taper 108 clearly visible in the softer primary buffercoating 100. In certain cases, the taper may also extend into thesecondary buffer layer as shown in FIG. 11. The taper 108 provides aconical boundary separating the bare optical fiber 106 from theoverlying buffer structure 100,102.

As described, the mechanical stripping apparatus includes two sets ofvertically opening and closing cutting blades 124, 130 adapted forvertical, then horizontal movement either independently orsimultaneously. A pair of clamps 120, 122, on either side of the cuttingblades 124, 130, holds a strippable filament in a taught conditionduring the stripping process. Another embodiment of a mechanicalstripping apparatus alters the angle of the incision during the cuttingprocess to modify the shape of a tapered transition 108. As the blades124, 130 close towards the coating 100, 102 around a clamped fiber 20 anangled surface or biasing cam surface deflects the path of the blades toa prescribed entry angle into the coating 102 so as to provide acontrolled tapered transition. This produces an intentionally angled cutby moving the blades 124, 130 diagonally into the coating. Any change inthe angle of the cam surface produces a corresponding change in theangle of a tapered transition 108 to allow consistently reproduciblecontours of a coating 100, 102 abutting either side of a bare portion ofan optical fiber. Suitable selection of the cam angle produces taperedtransitions 108 having contours and dimensions facilitating essentiallydefect-free recoating of bare optical fiber portions. Successfulmechanical stripping to provide a tapered transition may proceed underambient conditions, as indicated previously. With some buffers, however,the modulus of the buffer resin is in a range that complicates theformation of a tapered transition. In such cases, it may be necessary tosoften the resin by heat or chemical action before attempting themechanical stripping process to produce the desired tapered transition.

Completion of the mechanical process of fiber stripping leaves a centralportion of an optical fiber 20 having a central sleeve 110 of protectivebuffer coating 100, 102 that has been separated from the remainder ofthe buffer coating 100, 102 over the optical fiber core 106. Opposinggaps 132, 134 between the sleeve 110 and remainder of the buffer coating100, 102 provide points for hot acid to penetrate under the centralsleeve 110 to facilitate removal of the sleeve 110, which dissolves inhot acid. Preferably the acid does not reach the tapered transitions 108of a previously mechanically stripped coated optical fiber 20. Removalof the central sleeve 110, as a solution in hot acid, followed byrinsing in water and alcohol, leaves a clean, bare portion of an opticalfiber 106 in suitable condition for further processing. Depending on theeffectiveness of mechanical stripping, much of the disrupted buffersleeve 110 may be lifted from the bare fiber portion. This reduces theamount of buffer coating to be dissolved from a fiber 20 during acidstripping.

Stripping of protective buffer coating from an optical fiber 20 may beconducted as an automated or semi-automated process using equipmentsuitably designed for the task. Preferably design of the equipmentallows processing of multiple fibers 20 in a single operation. FIG. 17shows the positioning of a filament organizer 10 relative to amechanical stripping apparatus 150. A mechanical stripping apparatus 150according to the present invention includes a base 152 as a mountingplatform for optical fiber clamps 120, 122 and sets of cutting blades124, 130. Optical fiber clamps 120, 122 may either move relative to thebase 152 or be secured thereto. Optional securing of the clamps 120, 122facilitates mechanical stripping with a fiber 20 either at its originaltension, set by the filament organizer 10, or under tension produced bymoveable clamps 120, 122.

The sets of cutting blades 124, 130 slidably engage the surface of thebase 152. Slidable engagement of the sets of cutting blades 124, 130facilitates the axial movement of the blades 124, 130 to form a taperedtransition 108, as previously described. A filament organizer 10 may besuspended by any suitable method relative to the mechanical strippingapparatus 150 provided that the filament 20, clamps 120, 122 and cuttingblades 124,130 have alignment on a common axis.

After the preliminary step of mechanically stripping the central portionof a fiber 20, acid stripping may require formation of a loop, suspendedfrom a filament organizer 10. The suspended loop may be submerged in hotconcentrated sulfuric acid. An acid bath is a convenient and cleanmethod to remove the buffer coat from acid-resistant glass.

A known method uses acid to remove protective coatings from opticalfibers. The method requires handling of fibers each individually as muchas six to eight meters long. Handling of such optical fibers requirescaution because of the small diameter and transparency of thefilamentary structure. If the optical fiber snags an object duringhandling, the glass fiber core could fracture without showing immediateevidence of damage.

A manual method for loop formation includes extending an optical fiberover two blocks having a distance of separation of about six inches. Thefolding of a first block 160 over a second block 162 produces a loop164, shown by the diagram of FIG. 18. This figure also includes an acidbath 166 with capability to suspend the blocks 160,162 and loop 164 insuitable position for acid 168, preferably sulfuric acid, to dissolveprotective buffer coatings 100, 102 from the U-shaped loop 164. Thelength of coating 100, 102 removed from an optical fiber 20 will dependupon the depth to which the optical fiber loop 164 extends below thesurface 170 of the acid.

Loops having substantially a desired size and shape form relativelyeasily, but individual fibers need careful handling to avoid damage toexposed glass surfaces. The “U” shaped loop 164 of fiber 20 lies on oneside of a pair of blocks 160,162 with relatively long trailing fiberends 172 extending from the opposite side of the blocks 160,162. In thisarrangement the loop 164 and the fiber ends 172 are at risk for breakageor related damage. Damage occurs in different ways including inadvertentcontact or impact during fiber processing operations including acidremoval of buffer coating 100, 102 from the fiber 20, fiber Bragggrating writing, fiber annealing, fiber recoating and the like. Thisproblem may now be avoided using filament organizers 10 according to thepresent invention. Filament in the form of optical fiber 20 is readilyloaded onto filament organizers 10 without damage. Also the design of afilament organizer 10 allows stacking of multiple organizers to increaseprocess throughput. A stacked configuration 68 of filament organizersplaces filaments, i.e. optical fibers 20, in a suitably spaced-apartrelationship for processing from fiber stripping to fiber Bragg gratingrecoating, as further described below.

FIG. 19 indicates arrangement of a filament organizer 10 for acidstripping using an apparatus that will reposition an optical fiber 20 ina filament organizer 10 to produce a suspended optical fiber loopsimilar to the previously described loop 164 formation between blocks160, 162. The apparatus grips the coated optical fiber 20 at two pointsalong its length as indicated in FIG. 19. A loop forms when these twopoints move toward each other, as shown in FIG. 20. The fiber is held atone point between a first pair of rolls 180 and at a second point by asecond pair of rolls 182. A layer of elastomer covers a lower roll 184,188 of each pair of rolls 180, 182. Each roll diameter is based on theminimum advisable fiber bend diameter to avoid inducing damaging bendingstress during optical fiber manipulation to form a loop. The elastomerprovides compliance to lower contact stress, reduce fiber slip, and holdmultiple fibers simultaneously.

Each pair of rolls 180,182 converges to pinch the fiber 20. Next, theupper roll 186, 190 of each pair rotates toward each other, inducing ashallow bend in the fiber 20. The shallow bend establishes a plane to beoccupied by a machine-formed loop when the pairs of rolls 180, 182 movetoward each other. The looping method works with the fiber tray 10 byusing two removable upper rolls 186, 190 and two non-removable lowerrolls 184, 188.

The present invention, in one of its embodiments, facilitates theprocess of acid stripping of multiple fibers in a single operation.Successful processing of multiple fibers requires several features thatare possible using filament organizers according to the presentinvention. Of particular importance is the use of a tool that shapesoptical fiber into loops while minimizing the possibility of fiberdamage. A suitable loop-shaping tool produces loops with repeatable sizeand shape. Once formed, loops preferably do not bend out of plane. Thislatter feature is important to the processing of multiple fibers thatwould tend to interfere with each other if out-of-plane bendingoccurred. Also, loops formed using stacked configurations of multiplefibers possess substantially the same size and shape, as required forautomated and/or semi-automated processing.

FIG. 21 shows a perspective view of a loop former 200 according to thepresent invention including detail of the position of a stackedconfiguration 68 of filament organizers 10 inside a loop formingcontainer 202. The loop-forming container 202 is essentially a closedbox including a floor 204, a front wall (not shown), a rear wall 206 andfirst 208 and second 210 sidewalls. Inside the container 202, a firstledge 212 occupies a position adjacent the junction between the firstsidewall 208 and the floor 204. A second ledge 214 occupies a similarposition adjacent the junction of the second sidewall 210 and the floor204. The floor 204 includes a plurality of longitudinal slits 216disposed orthogonally towards the first 212 and second 214 ledges. Eachof the front wall and the rear wall 206 includes a generally U-shapedguide slot 218 that is indicated in dotted line form in FIG. 21.Preferably the guide slot 218 includes a horizontal slot 220 joined toan angled slot 222, at one end, and an opposed angled slot 224 at theother. A seated roller 226 includes an axle 228 engaging one end of thehorizontal slot 220 in the front and rear 206 walls of the loop-formingcontainer 202. The seated roller 226 preferably has a covering of anelastomeric material. A movable roller 230 includes axle ends 232extending into the horizontal slot 220 at the front and rear 206 walls.The movable roller 230 is repositionable along the length of thehorizontal slot 220 to facilitate formation of multiple extended loops234 from a stacked configuration 68 of filament organizers 10 positionedin the loop-forming container 202.

The loop former 200 provides accommodation for a stacked configuration68 of filament organizers 10. Installation of a stacked configuration 68inside a loop-forming container 202 requires orientation of the stackedconfiguration 68 to provide alignment of filaments 20 with the pluralityof slits 216 in the floor 204 of the container 202. This may beaccomplished by holding a stacked configuration 68 by the openings 26 insupport boards 12 followed by lowering the stacked configuration 68 intothe loop-forming container 202 until the mounting plates 22 of thefilament organizers 10 rest on the first 212 and second 214 ledges, asshown in FIG. 21. The stacked configuration 68 is installed with theseated roller 226 and movable roller 230 at opposite ends of thehorizontal slot 220. Correct positioning of the stacked configuration 68relative to the seated 226 and movable 230 rollers provides gentlecontact between each filament 20 and the surfaces of the rollers 226,230. In this condition, the filaments 20 should still be under tensionand free from bends.

Having positioned the stacked configuration 68 in the loop-formingcontainer 202 with the filaments 20 touching the fully separated rollers226, 230 a first rod 236 inserted through the angled slot 222, of thefront wall, extends across the container 202 to enter the angled slot222 in the rear wall 206 of the loop forming container 202. Afterinsertion, the first rod 236 occupies a step 238 of the angled slot 222.A second rod 240 similarly positioned, in a notch 242 of the opposedangled slot 224, completes formation of the first pair 180 and secondpair 182 of rolls. Movement of the first 236 and second 240 rods tofollow the contours of the angled slot 222 and the opposed angled slot224 initially establishes contact between the rods 236, 240 and thefilaments 20. An elastomer band stretched over the ends of each pair ofrolls 180, 182 draws the rolls together to increase their gripping forceon the filaments 20. With continued gentle urging, the rods 236,240continue movement towards the horizontal slot 220. This movement placesthe rods 236,240 side by side with the corresponding rollers 230, 226 ateach end of the horizontal slot 220. During this movement the filaments20 begin to wrap around the rollers 226, 230 in response to downwardforce applied by the rods 236,240. The applied force also draws filamentfrom each rotary spool 18 with the resulting formation of preformedloops having a height approximately equal to the diameter of the rods236, 240. Preformed loops become extended loops 234 of greater heightthrough movement of the first pair of rolls 180 towards the second pairof rolls 182. This creates an exposed loop 234 protruding through eachslit 216 in the floor 204 of the loop-forming container 202. By design,the slits 216 limit the amount of out-of-plane bending by the filaments20. Design features of each slit 216 include a loop entry 244 about 15.0mm (0.625 inch) wide, and a narrower loop station 246, having a width ofabout 1.6 mm (0.064 inch). The loop entry 244 is wider than the loopstation 246 to prevent contact of the mechanically stripped fiberportion of an optical fiber 20 with the sides or other parts of a slit216 during the initial stages of exposed, extended loop 234 formation.As the height of the loop 234 increases, the mechanically strippedportion of the fiber emerges below the floor 204 of the loop-formingcontainer 202. Any optical fiber 20 residing in a slit 216, at thispoint, has a covering of buffer coating to protect the optical fiberfrom contact with any of the slit 216 surfaces. Thus, protected, thelooped optical fiber 234 enters the narrow loop station 246 where itwill stay during removal of residual primary and secondary buffercoatings by acid stripping. The loop entry 244 opens to the upper andlower surfaces of the floor 204 of the loop-forming container 202.Preferably the opening to the upper surface of the floor 204 is wider(15.0 mm) than the opening (12.5 mm) to the lower surface of the floor204. This description indicates that the loop entry 244 of each slit 216has a somewhat V-shaped cross section to assist in controlling thespatial positioning of each exposed, extended loop 234. Loop controlprovided by each narrow loop station 246 counteracts torsional stressesintroduced during manufacture of filaments 20 in the form of opticalfibers. It will be readily appreciated that each extended loop 234 hangsbelow the floor 204 of the loop-forming container 202 ready forimmersion in an acid-containing bath 166 as indicated in FIG. 20.

FIG. 22 shows the condition of an optical fiber 20 after the processsteps of mechanical stripping, and acid stripping to provide a length ofan optical fiber 20 having a buffer-free bare central portion 250suitably prepared for refractive index modification associated with thewriting of a Bragg grating. Before the actual writing of a gratingoccurs, the unspooling of an end section of fiber provides an opticalfiber pigtail 252 suitable for the formation of splices or connectionsbetween optical fibers 20. At this point in the process, a pigtail 252at each end of the optical fiber, carried on the filament organizer 10,provides a point of connection so that the progress and accuracy ofgrating writing may be optically monitored during the writing process.

Optical fiber Bragg gratings may be written in a plurality of opticalfibers 20 each having a bare central portion 250. Convenient handling ofthese fibers 20 uses filament organizers 10 in a stacked configuration68 according to the present invention. The pigtails 252 of each opticalfiber 20 require positioning using fixtures to permit accurate alignmentof fiber ends with equipment that monitors the progress of fiber Bragggrating writing. The fixture is an optical fiber connector including acentral body with opposing fiber receiving ends. The monitoringequipment may use optical fiber or non-contact coupling to the pigtailends to complete the optical circuit. Connection to fibers 20 from eachfilament organizer 10 in a stacked configuration 68 uses pigtail endsadapted to plug into the connector. A connector may accommodate pigtailends of a single optical fiber 20 or a plurality of fibers 20 havingpigtail ends previously terminated to the requirements of a multi-fiberconnector. The term “ribbonized” has been applied to one form oftermination wherein the ends of pigtail sections of fiber lie side byside to form a single layer of fibers having a flat ribbon-likeappearance. Positioning of the pigtail ends allows them to mate with amulti-fiber connector.

FIG. 23 indicates a filament tensioning apparatus 260 used for furtherprocessing of an optical fiber 20 held in a filament organizer 10according to the present invention. After placement of an optical fiber20 on a support board 12, as discussed previously, the portion locatedbetween the mounting plates 22 of the filament organizer 10 exists in acondition of tension applied by a tensioner 58. In preparation for thewriting of a grating, the tension wire 62, of a selected filamentorganizer 10 (see FIG. 3), may be grasped in the vicinity of the notch66 and extended slightly parallel to the edge of the board 12, andtoward the nearest guide 24. Movement of about 1.0 mm to about 2.0 mmreleases the tension force applied to a filament 20 by the tensioner 58.Preferably the filament organizer 10 is one of a stacked configuration68 positioned on a platform of an indexing unit. The indexing unitraises and lowers the stacked configuration 68 using any one of avariety of mechanical and hydraulic structures. In one embodiment,sliding engagement of the platform with one or more vertical posts orbeams allows upward and downward movement of the platform and stackedconfiguration it supports. The platform may include organizing mountsfor mating engagement with spacer blocks on the lowest filamentorganizer so that the stacked configuration 68 is suitably aligned withthe optical fiber 20, of each filament organizer, accessible to a fibertensioning apparatus 260.

The indexing unit remains stationary during the approach of a fibertensioning apparatus 260 to apply clamps and grippers to a centralportion of each optical fiber 20 in a stacked configuration 68. Approachof the fiber tensioning apparatus 260 may be facilitated by an alignmentmechanism for optimum positioning between an optical fiber 20 and clamps262, 264 of a fiber tensioning apparatus. Optimum alignment does notnecessarily require attachment of the indexing unit to a fibertensioning apparatus 260.

Clamps 262, 264 attached at each end of the central load-free fiberportion 20 retain the central portion in its load-free condition, beforesubjecting the optical fiber to a selected tension force. The clampscomprise components of a fiber Bragg grating tensioning holder 266 usedto stretch the fiber 20 under a prescribed load during fiber Bragggrating writing. A tensioning holder 266 according to the presentinvention comprises a voice coil 268 as a load applicator to a load cell270 that measures the load applied between the pair of clamps 262, 264holding the central portion of the fiber 20. After precise applicationof a desired selected tension, a pair of grippers 272, 274 isolates ameasured fiber portion 276, between the clamps 262, 264, setting up atension zone independent of outside tension variations. This maintains aprescribed load on the measured fiber portion 276 and prevents any fiberslippage relative to the grippers 272, 274 and hence the clamps 262,264.The fiber Bragg grating may be written into the bare portion 250, of theisolated, measured fiber portion 276 held between the pair of grippers272, 274. Tension applied to a clamped optical fiber 20 anticipatesshrinkage that will occur, changing the separation between gratingfeatures after a grating has been written and after a piece of axiallystrained measured fiber portion 276 has been released from the pair ofgrippers 272,274.

A voice coil driven tensioning holder 266 is favored over any of severalpossible load applying units including a DC servo motor and encodercombination, a precision pneumatic cylinder, a high precisionmicro-positioning linear stepper motor and a mechanical balance beam. Aprecision pneumatic cylinder, for example, provides insufficient fibertension and fine pressure control to accurately apply a prescribedamount of tension to an optical fiber. A high precisionmicro-positioning linear stepper motor is equally incapable of providingrequired precise tension adjustment. Problems associated with the use ofa mechanical balance beam include the fact that it is primarily a manualprocess not particularly conducive to automation.

Voice coil activated clamping structures are known. For example, U.S.Pat. No. 4,653,681 describes a voice coil activated fine wire clamp,used in wire bonding applications. Clamp jaws may be moved to an openposition from a normally closed position using a voice coil motor undermicroprocessor control. A voice coil programmable wire tensioner,described in U.S. Pat. No. 5,114,066 also facilitates wire bonding. Thisshows that it's known to use a voice coil in wire bonding applications.However, it appears that the use of a computer controlled, voice coilmotor has not been used to apply repeatable, precise amounts of tensionto optical fibers for consistent production of optical fiber Bragggratings having essentially the same wavelength response.

The advantageous use of a voice coil actuator 268 provides a linearoutput force corresponding to an input current that may be finelycontrolled. A high precision power supply with a voice coil actuator 268produces a stable signal leading to an output force that is remarkablyconstant. This allows selection of a wide range of output force, limitedonly by the magnitude of energy transfer between a coil and a magnet.The output force of the actuator 268 is proportional to the inputcurrent, similar to a DC motor. A tensioning method based upon a voicecoil actuator 268 occurs in response to bearing-free passage of energybetween a coil and a magnet. Tension adjustment using this method offerssignificant advantages over prior methods that used addition and removalof static weights to increase or decrease tension on a fiber.

FIG. 23 shows that the mounts 278, 280 for the voice coil and load cellinclude air bushing carriages 282, 284 for minimal friction relative toa support bar 286. Air bushing carriages 282, 284 reduce static frictionin the system to a low, almost insignificant level. Reduction offriction in the bushings 282, 284 allows accurate application of fibertension corresponding to the force acting on the load cell 270. Thisresults in improved control of the force generated by the voice coilactuator 268 and more consistent application of tension to an opticalfiber 20. Each carriage 282, 284 includes a clamp 262, 264 forattachment to a central portion of an optical fiber 20. The separationbetween the clamps 262, 264 identifies the central portion of theoptical fiber 20 to be tensioned. An extending guide rod 288 attached tothe moving coil of the voice coil actuator 268 pushes against the loadcell 270 increasing separation between the two carriages 282, 284.Increasing separation between the carriages 282, 284 operates throughthe clamps 262,264 to move them away from each other to add strain tothe optical fiber 20. Upon attainment of a desired strain a pair ofgrippers 272, 274 grasp an inner measured fiber portion 276 of theoptical fiber 20. The measured fiber portion 276 is somewhat shorterthan the central portion between the clamps 262, 264. Accuratemaintenance of force at selected levels allows the writing of acceptablefiber Bragg gratings. Force selection and control relates to the use ofa high precision load cell 270 to measure and display the tensioninitially applied to the fiber 20 and maintained during the writingprocess. The load cell 270 may also provide feedback during computercontrolled automated fiber Bragg grating writing.

An important aspect of writing a fiber grating is the need to hold ameasured fiber portion 276 in a fixed, immobile condition throughout theprocess. This requires the use of a jaw assembly 290 attachedparticularly to the grippers 272, 274 for removably securing a measuredfiber portion 276 in the desired immobilized condition. Clamps 262, 264attached to the filament tensioning apparatus 260 may use the same jawassembly 290 or another providing adequate clamping of a central portionof a fiber 20.

FIG. 24 illustrates a fiber portion gripper 272 with an attached jawassembly 290 of suitable design. The jaw assembly 290 comprises a lowerjaw 292 attached at the end of a gripper 272 and an upper jaw 294 forengagement with the lower jaw 292 to grip and immobilize a measuredfiber portion 276, shown in cross section in FIG. 24.

FIG. 25 provides a detail drawing of a V-shaped channel 296 formed inthe upper surface of the lower jaw 292 and a rectangular cross sectiongroove 298 in the lower surface of the upper jaw 294. The sizing of eachof the channel 296 and groove 298 of the jaw assembly 290 corresponds tothe diameter of the measured fiber portion 276 that is held in animmobilized condition.

A jaw assembly 290 design requires matching of the dimensions of a fiber20 with those of a V-shaped channel 296 and a rectangular groove 298.Dimensional matching allows the application of substantially equalpressure at contact points around the circumference of a measured fiberportion 276 held immobile for the writing of a Bragg grating accordingto the present invention. Preferably a fiber 20 is held between theV-shaped channel 296 and the rectangular groove 298 with equal pressureapplied at points of contact around its circumference. This is indicatedin FIG. 25 by the fact that the two points of contact of the fiber 20with the V-shaped channel 296 and the fiber's point of contact with thegroove 298 are equidistant from the bare optical fiber 106. Uniformapplication of pressure leads to even distribution of stresses acrossthe diameter of a measured fiber portion 276 to reduce fiber damage to aminimum, considering the amount of pressure required for the grippers272, 274 to hold the measured fiber portion 276 in an immobilecondition. V-groove chucks are known for clamping portions of opticalfibers, as taught by U.S. Pat. Nos. 4,623,156 and 5,340,371. It does notappear in either case that consideration is given to equalizing theamount of pressure applied to points around the circumference of afiber.

In a preferred embodiment of the present invention, pressureequalization around the circumference of an optical fiber 20 requiresthe use of a floating upper jaw assembly 295, as shown in FIG. 26. Theself-adjusting, floating upper jaw assembly 295 comprises a fiber clasp300, a support flange 302, and an angular compensator 304 (see FIG. 27)separating the fiber clasp 300 from the support flange 302. A fiberclasp 300 may also be referred to herein as a filament clasp. Eachgripper 272, 274 includes, in this case, an upper jaw mount 306 and alower jaw mount 308. A lower jaw 292 attaches to the lower jaw mount 308and an upper jaw assembly 295 attaches to the upper jaw mount 306 by thesupport flange 302. Suspension of a fiber clasp 300 from a supportflange 302 preferably uses a spring-loaded connector 310. Spring tensionoperating between the fiber clasp 300 and support flange 302 retains anangular compensator 304 between them. During capture of a filament 20between the lower jaw 292 and upper jaw 294 of a gripper 272, 274, theuse of a floating upper jaw assembly 295 allows application of grippingforce to a filament 20 substantially without displacement or rotation ofthe filament 20. The clamps 262, 264 may also include a floating jawassembly.

FIG. 27 shows the result of gripping a filament 20 between a floatingjaw assembly 295 and a lower jaw 292. As the floating jaw assembly 295moves towards the lower jaw 292, the rectangular groove 298 of thefilament gripper 292, 294 makes contact with a filament positioned inthe V-shaped channel 296 of the lower jaw 292. As contact occurs, thefilament clasp 300 may adjust slightly to apply gripping force uniformlyto the filament 20, without disturbing it. Adjustment of the filamentclasp 300 relies upon its independent movement due to the angularcompensator 304 that separates it from the support flange 302. Apreferred angular compensator 304 according to the present inventioncomprises a spherical element that prevents contact between the filamentclasp 300 and the support flange 302. Preferably the angular compensator304 seats between a substantially conical shaped depressed portion 301in the fiber clasp 300 and a substantially conical recess 303 in thesupport flange 302. The angular compensator 304 maintains separation ofthe support flange 302 from the filament clasp 300 to allow them to moveindependently. Also, the spherical structure of the angular compensator304 allows effective change of angle around the perimeter of thefilament clasp 300.

The previous discussion provided a description of positioning, clampingand gripping a single optical fiber 20 using an apparatus 260 includinga tensioning holder 266 to tension the fiber 20 during writing of aBragg grating. The description involves the relative positioning betweena filament organizer 10 and a tensioning holder 266. When a filamentorganizer 10 represents one of a number of organizers 10 in a stackedconfiguration 68 the writing of a Bragg grating may be accomplished in avariety of ways. For example, fiber optic Bragg gratings may be writtenone at a time using a step and repeat process to move a fiber 20 carriedin a selected filament organizer 10 into the correct position, relativeto the tensioning holder 266 to execute writing of a Bragg grating. Thewavelength response of an optical fiber 20 may be monitored, asdescribed previously, during Bragg grating writing. An alternative tosequential writing of Bragg gratings may be to use a bank of tensioningholders 266 and related writing devices for producing a plurality ofBragg gratings simultaneously.

The step and repeat process using an indexer to reposition a stackedconfiguration 68, e.g. preferably by up or down directional movement,presents a new fiber to the Bragg grating writing device. The stackedconfiguration 68 fits into the platform of an indexer adapted to providemating with a known positional relationship between a stackedconfiguration 68 and the platform using alignment between the spacerblocks 70 and organizer mounts 72. Having established the preferredplacement of the stacked configuration 68 relative to the indexer, andhaving made connection of the fiber pigtails 252 to the opticaldetection system, a scan of each fiber verifies the existence ofreliable optical connections.

The placement of a stacked configuration 68 in an indexer with fiberoptic connection to an optical detection system precedes seriate Bragggrating writing process in which the indexer initially uses an opticalsensor to scan the filament organizers 10, counting the number in thestacked configuration 68. This process designates the first filamentorganizer 10 in the stacked configuration 68. A sequence of operationsmodifies the optical fiber 20 held in this first filament organizer 10.Before modifying the optical fiber itself, removal of the effect of theconstant force tensioner 58, as described previously, uses a forcereduction assembly comprising a pair of pulleys 64 on either side of anotch 66 formed in an edge of the support board 12. The tension wire 62,passing around the pulleys 64 and across the notch 66, may be grasped inthe vicinity of the notch 66 and extended slightly, parallel to the edgeof the board 12, and toward the nearest guide 24. This releases thetension on the rotary spool 18 of the filament organizer 10, therebyreleasing the tension in the filament or optical fiber 20.

Preparation for modifying a filament 20, in the form of an opticalfiber, requires securing the tension-free portion of the optical fiberusing an apparatus that combines a filament tensioning apparatus 260 andan interference pattern generator (not shown). Each of the filamenttensioning apparatus 260 and the interference pattern generator may bemoved separately initially to secure and position an optical fiber 20,as discussed previously, and then to modify the fiber's structure.

The filament tensioning apparatus 260 grips the fiber 20, as describedabove, using a first clamp 262 and second clamp 264. Force operatingbetween the two clamps 262, 264 applies tension to the portion ofoptical fiber 20 between them. The force may be generated using a voicecoil actuator 268. The amount of tension is predetermined and measuredusing a load cell 270. At this point the optical detection systemprovides a reference scan of the optical fiber 20, including the portionheld under tension between the clamps 262, 264.

To reproducibly modify an optical fiber 20, preferably a measuredportion 276 of the fiber 20 remains in a fixed condition held by a firstgripper 272 and second gripper 274 that grip the fiber and hold it. Oncethe measured portion 276 of the optical fiber has been immobilized, aninterference pattern generator moves into close proximity to themeasured portion 276 of the optical fiber 20. Light, from a containedlaser source, passes through an opened shutter, and an optical system,including the interference pattern generator to produce an interferencepattern. The proximity of the interference pattern generator to theoptical fiber 20 provides sufficient energy to reproduce the linecharacteristics of the interference pattern or interferogram in the core106 of the optical fiber 20, preferably within the measured fiberportion 276. Impingement of actinic radiation, produced by anultraviolet laser, produces an optical fiber Bragg grating as a resultof changes in refractive index in parts of the optical fiber core 106affected by the radiation. Refractive index modulation corresponds tothe pattern of the interferogram, produced by the interference patterngenerator. Progress in reproduction of an interferogram in the core ofan optical fiber may be monitored using an optical detection system fordata acquisition. Data acquisition follows changes in the transmissionspectrum produced by a developing Bragg grating with time. Upon sensingthe desired transmission spectrum, the optical detection system closesthe shutter to prevent further exposure of the optical fiber to laserlight.

Following completion of optical fiber modification and removal of theinterference pattern generator from the vicinity of the measured fiberportion, the grippers 272, 274 and clamps 262, 264 retract from thefiber 20 to allow the filament tensioning apparatus to move to the nextfilament organizer 10 in the stacked configuration 68. Once separationof filament organizer 10 from the Bragg grating writing equipmentoccurs, the force reduction assembly releases the optical fiber placingit once again under the tension generated by the tensioner 58 of thefilament organizer 10. This completes the modification of a givenoptical fiber so that the indexer can readjust to align the opticalfiber 20 in the next filament organizer 10, in a stacked configuration68, with the filament tensioning apparatus and the interference patterngenerator before repetition of the Bragg grating writing cycle.

Annealing using an annealing oven at 300° C. for 10 minutes providesstabilization for a Bragg grating produced by refractive indexalteration of an optical fiber. An annealed Bragg grating may requireprotection by recoating the central portion of the optical fiber, whichwas previously stripped of protective coating. Any of a number ofmethods may be used for protective recoating of optical fiber Bragggratings including in-mold application, extrusion coating and spraycoating a fiber with a curable liquid coating. Equipment is commerciallyavailable for in-mold application of liquid recoat formulations. Thequality of in-mold optical fiber section recoating varies with the skillof an operator to carefully position a fiber in a mold cavity. Also,product yields have been reduced because of coating defects and fiberstrength issues associated with fiber handling and sectional recoating.As alternatives, either spray coating or extrusion coating may be usedfor recoating optical fibers that include Bragg gratings according tothe present invention.

A filament organizer 10 according to the present invention may be usedto advantage for positioning uncoated portions 250 of and optical fiber20 in a fiber recoating mold. Since the filament organizer 10 alsoapplies tension to the optical fiber 20, an alignment plate attached toa mold recoater of the type supplied by Vytran Corporation ofMorganville, N.J. is the only requirement for correct fiber positioningwithin a groove such as a semicircular or V-groove of the split moldapparatus. The alignment plate may use strategically positioned studs toengage the through holes 80 of the planar support 12 of a filamentorganizer 10. This eliminates the need for mold positioning usingmicromanipulator platforms and the like. The effective diameter of thegroove is somewhat greater than that of the remaining coated portions ofthe fiber. Due to pre-tensioning of the fiber by the filament organizerthe common need for external tensioning is eliminated. Once thevulnerable uncoated portions 250 of the fiber 20 have been suspendedclear of the groove surface, the hinged mold is closed and recoatingmaterial is injected into the groove until it extends to the coatedportion of the fiber. The molding material is then cured yielding arecoated section with dimensional characteristics essentially identicalto those of the original coated fiber.

Fiber recoaters of the type described briefly above include a splitsteel mold, each portion of which contains a matching semicirculargroove to accommodate the fiber. The grooves, when clamped together,formed a cylindrical bore slightly larger than the coated fiber OD topermit escape of air during injection of the coating material. Theoriginal coating in this arrangement keeps the uncoated sectionsuspended in the bore. A short uncoated length of fiber, typically nomore than half an inch, minimizes the possibility of damage throughcontact with the bore. Also, a series of clamps, attached on either sideof a central fiber portion, prevent the uncoated portion from touchingthe bore. Before injecting recoating fluid, the upper half of the moldis clamped in position to form the cylindrical bore. The curablerecoating fluid may be a room temperature curing epoxy resin or otherresin that cures either at elevated temperature or in response tosuitable radiant energy such as ultraviolet radiation.

FIG. 28 illustrates the use of a filament organizer 10 to store anoptical fiber 20 having a bare portion 250 that has been modified toinclude a Bragg grating. An exposed Bragg grating may be recoated afterpositioning the filament organizer 10 in a suitable spray recoatingapparatus 320. For correct positioning of a filament organizer relativeto a spray recoating apparatus 320 the bare portion of an optical fiber250 lies in the path of spray ejected from a recoating spray head 322.Such correct positioning is achievable by any of a variety of methodsand devices. One such method uses a plate suitably positioned relativeto a spray recoating apparatus and including alignment studs to engagethrough holes in a filament organizer 10 to place the bare portion 250of an optical fiber 20 in the optimum position for application ofrecoating spray.

A spray recoating apparatus 320 comprises at least one recoating sprayhead 322 and a radiation source 324. A filament organizer 10 is adaptedfor oscillatory movement of the bare portion 250 of an optical fiberbetween the recoating spray head 322 and the radiation source 324.Preferably, the position of the recoating spray head 322 is from about 1to about 2 from the fiber 20, preventing contact between the spray head322 and a deposited coating. The spray recoating method providescontrolled sectional recoat that achieves performance characteristicsnot obtainable from conventional in-mold recoating processes. It is anon-contact method since the optical fiber 20, including the bareportion 250, does not touch any part of the recoating equipment. Thisrepresents another benefit of suspending a fiber 20 in a filamentorganizer 10 that may be readily attached to the recoating apparatuswith precise fiber 20 to spray head 322 alignment. Another benefit ofspray recoating involves over coating one recoating composition withanother exhibiting different properties to produce a multilayer bufferstructure, around a fiber, including layers that differ in propertiessuch as modulus and durability or hardness.

The use of a spray recoating process allows flexible placement of asingle filament or multiple filaments in the path of spray or mist froma recoating spray head 322. Where a filament organizer 10 provides thepreferred means for handling an optical fiber 20, several filamentorganizers 10 may be closely located with variable orientation to placea plurality of fibers in the path of a single spray or directed mist. Anadditional advantage of spray recoating versus conventional cavity-moldrecoating is the provision of a recoating spray head 322 that may beadjusted or translated to differing lengths of bared optical fiberportions 250.

As the bared portion 250 of a fiber 20 traverses the location of therecoating spray head 322, one side of the bare fiber portion 250receives a light deposit of droplets from a mist of a curable recoatingcomposition. Movement of the filament organizer then places the depositof droplets in the illumination path of the radiation source 324. Theradiation cures the layer of recoating composition. Returning to thelocation of the recoating spray head 322, the filament organizer 10flips over to expose the opposite side of the previously bare fiberportion 250 to the spray of curable recoating composition. This allowsapplication of a fine mist of recoating composition to the exposedoptical fiber surface. This layer may be cured as described previously.Repeated processing by coating and curing with oscillation and flippingof the filament organizer 10 protects the fiber with multiple layers ofrecoating composition. The recoated fiber surface has a matte appearanceresulting from the build up of successive layers of coating material.Coating topography up to about 15 μm was revealed on the surface of amicroscope slide by surface scanning with an ELFA STEP mechanical stylusprofilometer available from Tencor Corporation.

Approximately fifty applications of recoating composition followed bycuring, after each pass, provide a layer having a thickness over therecoated length similar to that of the original buffer coatings on otherparts of an optical fiber 20. This technique allows layers of recoatingcomposition to be applied to the surface of an optical fiber to build aprotective recoat having a thickness of from about 10 microns to about100 microns on a bare fiber 106. The diameters of spray-recoated opticalfibers may be measured using a microscope and a QUADRA-CHEK 2000, fromMetronics Inc., Bedford, N.H. Coating thickness may be varied dependingon the application.

Another embodiment of the present invention provides a second recoatingspray head 326 and optionally a second radiation source 328 positionedopposite the previously discussed recoating spray head 322 and radiationsource 324. The description of multiple spray heads 326 and radiationsources 328 as occupying opposing or staggered opposing positionsincludes alignment of positions but is not limited thereto. Any numberof spray heads, positioned strategically, may be used in a fiberrecoating process. Placement of a spray head and radiation source onboth sides of an optical fiber 20 facilitates recoating of both sides ofthe bare fiber portion 250, while eliminating the need to flip thefilament organizer through 180°. As indicated previously, the use ofadditional radiation sources 328 is optional since the beam from asingle radiation source 328 may be directed to effect curing around thecircumference of a recoated fiber.

The contours of a deposit of droplets applied to a bare fiber 106 willreflect the size and shape of the droplet cloud issuing from a sprayhead 322. If required, a means for shaping the droplet cloud couldproduce a desired pattern of droplets on the surface of a fiber 20.Suitable shaping means include stencils, other types of masking devices,and stream deflectors such as air knives.

A preferred recoating process according to the present invention uses anair knife to direct an atomized stream at various angles of contact withan optical fiber 20. Air knife adjustment of the shape of a dropletcloud, and its angle of impingement with an optical fiber 20, may allowthe use of a minimum of spray heads 322, 326 to achieve optimum fiberrecoat uniformity and concentricity. Also, the use of air knifedeflection of small volumes of recoating compositions provides anadvantage when compared to the control of diverging streams ofrelatively high volume spray heads described in Japanese patent JP60-122754. U.S. Pat. No. 5,219,120 teaches the use of an air horn thatprovides a moving sheet of air to entrain a substantially uniform lineardispersion of atomized fluid moving above and extending substantiallyacross the width of the air horn. The air horn spreads the dispersion ofatomized fluid to a width suitable for spraying the flat surface of acircuit board. Such extensive spreading of a cloud of droplets does notapply directly to narrow curved surfaces such as those of an opticalfiber. Also, the air horn described in U.S. Pat. No. 5,219,120 is aseparate structure from the fluid atomizer.

Preferably air knife deflection according to the present inventionoccurs through the use of an air knife attachment that fits over theexit nozzle of a spray head. The air knife attachment includes a pair ofreceiving chambers, at least one on either side of the spray head, intowhich air may be directed. Each receiving chamber has an air entry atone end connected to an air reservoir. The opposite end of each chamberincludes an air knife slit that exits from the chamber at an angle tothe axis of the spray head. Air issuing from an air knife slit deflectsthe spray cloud, generated e.g. by an ultrasonic atomizing spray head,at an angle corresponding to the angle formed between the slit and theaxis of the spray head. Independent operation of each air knife,described above, causes selective deflection of a spray cloud at anangle that directs the droplet cloud towards an uncoated portion of anoptical fiber. Selective deflection of a droplet cloud allowspositioning of a number of optical fibers around a spray head nozzle.Impingement of air from exit slots of air receiving chambers deflectsatomized spray at various angles for sequential recoating of the numberof optical fibers held around the spray head using filament organizers10 according to the present invention. The use of air deflectionpreferably requires that the recoating composition is not oxygeninhibited. This does not prevent the use of oxygen inhibited recoatingfluids providing an inert gas is connected to the receiving chambers ofthe air knife attachment.

The process of recoating a bared portion 250 of an optical fiber 20 mayuse spray heads 322, 326 based upon either ink jet or ultrasonicatomization technology. Preferably, the application of curable recoatingcomposition, to an optical fiber 20, uses ultrasonic atomizationtechnology to dispense small particles (<50 μm) of a fluid, having aviscosity from about 40 to about 900 centipoises, preferably 40centipoises to about 400 centipoises, over a bared portion 250 of thefiber 20. Viscosity measurements were made at 25° C., using a BOHLINCS-50 rheometer. Other requirements for a coating composition forrecoating optical fibers according to the present invention depend uponthe intended use of a recoated optical fiber device, such as a Bragggrating. Example 1 of Table 1 provides a load bearing coating,preferably having a high modulus, high glass transition temperature(Tg), and temperature stability above the upper operating temperaturefor a selected application. Examples 2 and 3 produce cured coatings thatflex and bend with a recoated portion of a fiber. Preferably coatingcompositions, in this case, possess thermomechanical properties similarto undisturbed buffer coating, originally applied to the fiber.Immediate curing of such a coating reduces undesirable agglomeration,which could result in beading or poor concentricity.

An ultrasonic atomization processes differs from a spray atomizationprocess that, requires air velocity to break up a sprayable compositioninto droplets. Droplet size of a spray atomization process is larger (50to 100 microns diameter) and the spray velocity, at its lowest pressureof ˜20 psi, propels the droplets with a force causing the droplets tospread upon impact with a fiber surface. Being relatively high, theimpact force of an air atomized spray against a fiber causes build-up ofagglomerated droplet beads accompanied by formation of a non-concentriccoating.

The ultrasonic atomization process generates volumes of coatingcomposition that are extremely small, in the range from about 0.001ml/min to about 0.010 ml/min using a 2.0 cc glass syringe available fromPopper & Sons. The flow rate for dispensing a substantiallynon-directional cloud of droplets less than 50 microns in diameterdepends upon the speed at which the fiber is scanned in front of theatomizer head. A low velocity flow of nitrogen, or other inert carryinggas directs the cloud of ultrafine droplets of recoating compositiontowards a target surface. The low cloud volume and extremely smalldroplet size cause the formation of a textured discontinuous covering ofthe fiber surface. Although coatings are low enough in viscosity forspray application, preferred coating compositions exhibit minimal flow,after application, prior to coating. Flow and droplet agglomeration isfurther limited because the recoating composition, immediately afterapplication, undergoes exposure to curing radiation from the radiationsource 324, 328. Repeated application of recoating composition builds upa protective coating over a bared optical fiber portion 250. A recoatedoptical fiber preferably has a relatively smooth appearance bubble-freeappearance. This requirement guides the selection of materials used toprepare recoating compositions according to the present invention.

Suitable recoating compositions include low molecular weight, lowviscosity epoxy functional, 100% solids resins that photocrosslinkpreferably via an ionic mechanism initiated by a cationicphotoinitiator, especially an iodonium salt photoinitiator. Suchcoatings have good adhesion to the unstripped buffer coats on a fiber aswell as to the bare surface of the fiber. Ionic curing occurs withoutexclusion of oxygen. Radical curing recoating compositions may also beused in an inert environment. Suitable radiation sources forphotocrosslinking include those having wavelength emission in theblue/visible and ultraviolet wavelength regions of the spectrum. Curedcoatings according to the present invention.

A typical cured recoating composition has an elongation at least equalto and preferably greater than that of glass, i.e. more than 7%. Also, acured recoating composition has toughness and sufficient adhesion toglass to withstand accidental rubbing or contact with other objectsduring handling of a recoated fiber.

TABLE 1 Filament Coating Formulations Material* Example 1 Example 2Example 3 Weight % Epoxy A 57.0 — — Weight % Epoxy B 38.0 — — Weight %Epoxy C — 67.0 66.5 Weight % Epoxy D — 25.1 28.5 Weight % PolyetherGlycol —  2.9 — Weight % Iodonium Salt  5.0  5.0  5.0 Solution *Key:Epoxy A is CYRACURE UVR-6105 available from Union Carbide Corporation.Epoxy B is HELOXY 107 available from Resolution Performance Products.Epoxy C is EPONEX 1510 available from Resolution Performance Products.Epoxy D is HELOXY 7 available from Resolution Performance Products.Polyether Glycol is TERATHANE 650 available from E. I. du Pont deNemours and Company. Iodonium salt solution is UV 9380C available fromGeneral Electric Company.

Measurement of Coating Composition Viscosity

A Bohlin Model CS-50 controlled stress rheometer was used to measure theviscosities of coating compositions, for recoating filaments accordingto the present invention. The test method uses parallel plate geometryand “stress viscometry” mode. Viscosity measurement begins withplacement of a coating composition on the base surface of the parallelplate geometry. A second surface, mounted to rotate on a spindle, islowered into contact with the coating composition until a specified gapexists between the surfaces of the parallel plate geometry. Rotation ofthe spindle raises the rate of rotation to a number of revolutions perminute to produce a predefined stress (torque). The calculation ofviscosity values includes consideration of the geometry of the surfaces,the torque and the gap. Viscosities cited herein were obtained at 25° C.using a surface diameter of 20 mm, a gap between surfaces of 0.3 mm, anda stress of 93.8 Pascals.

A spray head that included an ultrasonic atomizer was used to applycurable recoating formulations, shown in Table 1, to the bare surfacesof several samples of silica fiber, each having a diameter of about 125microns. Each formulation was dispensed via the tip of the atomizinghorn of an ultrasonic atomizer available from Sono-Tek. The power supplyof the ultrasonic atomizer was set to a level of 5.4 watts. Successfulatomization of recoating formulations, having viscosities in the rangefrom about 40 centipoises to about 400 centipoises was achieved using amicro-bore fluid delivery tube through the center of the nozzle body ofthe ultrasonic atomizer. Most preferably the coating composition has aviscosity of about 200 centipoises. Recoating formulations were suppliedto the micro-bore tube at a syringe pump delivery rate of 0.015 ml/min.A preferred method uses a 21.5 gauge micro-bore tube available fromSmall Parts Inc., Miami, Fla. This provides precise control of smallvolumes of recoating composition delivered to the point of atomization.

Ultrasonic atomization as described previously produces anon-directional mist of coating composition that needs to be entrainedin a directional gas stream. Preferably the directional gas streamcomprises an inert gas, e.g. nitrogen gas, under the control of a shroudaround the micro-bore tube. A nitrogen gas stream flowing through theshroud around the atomizer head at a rate of 1.0 liter/min yields asuitably controlled atomized mist of recoating formulation. Adjustmentof the air shroud alters the contours of the gas stream therebymodifying the size, shape and coverage of a stream of droplets ofcurable recoating formulation impinging on a selected surface. Acontinuous coating may be formed on a surface using as few as about 4 toabout 6 applications of a coating formulation. However, depending uponprocess conditions, application of coating formulation may need to berepeated form about 40 to about 60 times to build a coating thickness ofup to 250 microns on a selected surface.

A filament recoating formulation was shown to produce a suitable streamof material for application using an ink jet printing/spray head asfollows:

EXAMPLE 4

Epoxy A 76.0 weight % Epoxy B 19.0 weight % Photoinitiator solution  5.0weight %

The photoinitiator solution contains 40 parts or iodonium methide, 60parts of decyl alcohol and 4 parts of isopropylthioxanthone.

The ink jet printing/spray head operated at a head temperature of 70° C.A ink jet printing/spray head, available from Trident InternationalInc., Brookfield, Conn. was selected to apply recoating composition toseveral samples of silica fiber, each having a diameter of 125 microns.The print head has 64 nozzles, each 50 microns in diameter. Use of afilament organizer mounted on a filament recoating apparatus providedsuitable alignment of a fiber with an ink jet printing/spray head priorto application of recoating composition. Particles of the compositionwere jetted over a 1 cm length, on one side of a filament of each offive samples of silica fiber. An EFOS ULTRACURE radiation source (EFOSInc., of Mississauga, Ontario, Canada), with an ultraviolet radiationwand, was used to direct energy to the coated sample to initiate cure.Repeated passes under the recoating spray head, followed by ultravioletradiation curing, produced adequate coverage of the bare optical fiber.

FIG. 29 provides a diagrammatic representation of a preferred split dieextrusion coating method that uses an recoating fluid extrusionapparatus 330 having a die head assembly 332 encircling an optical fiber20 to recoat a bared optical fiber portion 250 that contains a Bragggrating. A study, reported in Electronics Letters Vol. 34, No. 12, Jun.11, 1998, investigated a split die recoating process to apply a solutionof polyimide to a bare portion of an optical fiber. The process involveddrawing a fiber through the fluid filled split die, then driving offsolvent at 70° C. followed by baking the polyimide recoated section at300° C.

Split die extrusion coating according to the present invention offersimprovements for fiber recoating including controlled application andrelatively low temperature curing of recoating compositions as follows.The die head assembly mentioned above comprises a split sizing die 334and an in-line radiation cure chamber 336 that is closed around theoptical fiber 20. Accurate fiber 20 positioning, for recoating andprotection of the Bragg grating occurs during engagement of a filamentorganizer 10 with the recoating fluid extrusion apparatus 330. Any oneof a variety of methods may be used for engagement between a filamentorganizer 10 and an extrusion apparatus 330 provided that the die headassembly 332 has movable alignment to deposit a substantially uniformlayer around the fiber portion 250 that needs recoating. Duringrecoating, the bared fiber portion 250 of an optical fiber 20 remainsstationary between fiber positioners. The split sizing die 334 liesadjacent to one end of the bare fiber portion 250 from which curablerecoating composition will be applied across the remainder of the bareportion 250. Photocurable coatings extrude from the leading edge of thesizing die 334 as it traverses the length of the bared optical fiberportion 250. The radiation cure chamber 336 moves with the sizing die334 following behind it to initiate curing of the photocurable recoatingcomposition immediately after its deposition on the surface of theoptical fiber 20. The recoating composition curing reaction preferablyrequires an inert atmosphere. For this purpose an inert gas deliverytube 338 directs a flow of nitrogen into the radiation cure chamber 336that is illuminated using a suitable source of radiation, preferablyultraviolet radiation.

A linear transport mechanism 350 adjacent to the coating head 332includes a guide rod 352 and a carriage 354 slidably mounted on theguide rod 352 for movement along the guide rod 352. A connecting rod 356from the carriage 354 to the coating head 332 provides lineardisplacement of the coating head assembly 332 during movement of thecarriage 354 to move the coating head 332 from the first boundary to thesecond boundary of a bare portion 250 of an optical fiber 20. Curablefluid may be extruded from the sizing die 334 and energy from theradiation source 336 used to cure the fluid during recoating of the bareportion 250 of an optical fiber 20.

During its motion, the split die 334 applies a substantially uniformthickness of recoating composition along a length of fiber 20 thatincludes the bare fiber portion 250 and margins at each end that overlapthe original secondary buffer 102. Uniform coverage of an optical fiber20 with a concentric layer of a recoating composition relies upon theaccuracy of positioning a filament organizer 10 to preferably place thefiber 20 coaxial with the sizing die 334. The radiation cure chamber 336has a size such that its internal surfaces do not touch the layer ofrecoating composition, either before or after curing. When coatingconcentrically, the bare fiber portion 250 will only come into contactwith the recoating composition. The split configuration of the sizingdie 334 and the radiation cure chamber 336 allows easy positioning of afiber 20 in a recoating fluid extrusion apparatus 330. Correct fiberpositioning, as mentioned previously, is a result of accurate engagementof a filament organizer 10 with a recoating fluid extrusion apparatus330. Upon re-opening the die head assembly 332, after completing therecoating and curing process, a gap between a recoated fiber 20 and theinternal surfaces of the radiation cure chamber 336 allows clean removalof the fiber 20 from the assembly 332.

Changes in the length of bared fiber portions 250 may be accommodated byadjustment of the distance that a die head assembly 332 may travel whileextruding recoating composition. The surface tension of the recoatingcomposition tends to smooth out any irregularities in the coating beforeit reaches the radiation cure chamber 336, even though the die headassembly 332 has a length of only about 6.0 mm to about 7.5 mm. Abenefit of this short length is avoidance of contamination by recoatingcomposition. Also small amounts of residual recoating composition may berelatively easily cleaned from inside the assembly 332.

Although a bare fiber portion 250 has a horizontal orientation duringapplication of protective recoating composition, the moving extrusiondie 334 produces similar results to coating heads operating verticallyduring fiber draw coating processes. The relative motion between thesizing die 334 and the fiber 20 simulates the draw process. Thiseliminates mold recoating defects such as flash, gate marks, sinks, andcoating delamination caused by coating adhering to the surface of amold.

The extrusion of terminal margins, at each end of the bare fiber portion250, means that initial deposit of material by extrusion occurs at aregion of the fiber 20 that is still protected by the original coating100, 102. This substantially prevents optical fiber strength lossesgenerally associated with loading the fiber 20 into a traditionalrecoating mold. Bared fiber portions 250 recoated by split die extrusionaccording to the present invention provided evidence of strengthretention by surviving Vitran proof testing to levels exceeding 800kpsi.

A process for manufacturing an optical fiber Bragg grating has beendescribed to show how a compact filament organizer 10 may be used tohandle and transport optical fibers 20 between various types ofprocessing equipment. Each piece of processing equipment may include apair of mounting pins for alignment and insertion in through holes 80 ofa filament organizer 10 for correct positioning of a central portion ofan optical fiber 02 relative to the selected piece of apparatus. Sucheasy positioning also facilitates automation of at least parts of theBragg grating manufacturing process unlike previous similar processesthat rely upon operator skill for correct fiber positioning. It will beappreciated that engagement between mounting pins and through holes isonly one of a number of methods for aligning an optical fiber forprocessing.

As required, details of the present invention are disclosed herein;however, it is to be understood that the disclosed embodiments aremerely exemplary. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as abasis for the claims and as a representative basis for teaching oneskilled in the art to variously employ the present invention.

What is claimed is:
 1. A filament holding fixture having a closedposition for applying substantially equal pressure to a plurality ofpoints around the circumference of a filament, said holding fixturecomprising: a first jaw having a planar surface and an open endedchannel formed in said first jaw opening to said planar surface; and asecond jaw, engaging said first jaw to move said holding fixture betweena retracted position and said closed position, said second jaw includesan upper jaw mount and an upper jaw assembly comprising: a supportflange attached to said upper jaw mount; a fiber clasp having a contactsurface opposite a structured surface including an open ended groove ofsubstantially rectangular cross section; and a plurality of springconnectors to hold said fiber clasp to said support flange, said channeland said groove sized for even application of pressure to the filamentplaced therebetween, when there is axial alignment of said channel withsaid groove, and said planar surface lies adjacent said structuredsurface, said filament holding fixture being in said closed position. 2.The filament holding fixture of claim 1, wherein said channel has aV-shaped cross section.
 3. The filament holding fixture of claim 1,wherein said closed position prevents axial and rotational movement ofthe filament.
 4. The filament holding fixture of claim 1, wherein saidfirst jaw includes a lower jaw mount and a lower jaw including saidplanar surface and said open-ended channel formed therein.
 5. Thefilament holding fixture of claim 1, wherein the filament is an opticalfiber.
 6. The filament holding fixture of claim 1 wherein said supportflange includes a support surface having a substantially conicalrecessed portion formed therein and said fiber clasp includes a contactface having a substantially conical depressed portion formed therein. 7.The filament holding fixture of claim 6 further including an angularcompensator confined between said recessed portion and said depressedportion by force produced by said plurality of spring connectors, saidangular compensator maintaining separation between said support flangeand said fiber clasp to allow independent movement of one from theother.
 8. A filament holding fixture comprising: a gripper having anopen position and a closed position, said gripper further comprising: alower jaw mount; a lower jaw connected to said lower jaw mount, saidlower jaw having a planar surface and an open-ended, V-shaped channelformed therein opening to said planar surface to receive at least aportion of a filament; an upper jaw mount; an upper jaw assemblycomprising: a support flange attached to said upper jaw mount saidsupport flange including a support surface having a substantiallyconical recessed portion; a fiber clasp having a contact face opposite astructured surface including an open-ended groove of substantiallyrectangular cross section, said contact face having a substantiallyconical depressed portion formed therein, said open ended groove andsaid V-shaped channel in longitudinal alignment to contact at least aportion of a filament when said gripper is in said closed position; aplurality of spring connectors to hold said fiber clasp to said supportflange; an angular compensator confined between said recessed portion ofsaid support surface and said depressed portion of said contact face byforce produced by said plurality of spring connectors, said angularcompensator maintaining separation to allow independent movement of saidsupport flange from said fiber clasp for fine adjustment of said fiberclasp to apply substantially equal force at points of contact of saidopen-ended groove and said V-shaped channel with a filament heldtherebetween following movement of said gripper from said open to saidclosed position.
 9. A filament holding fixture having a closed positionfor applying substantially equal pressure to a plurality of pointsaround the circumference of a filament, said holding fixture comprising:a first jaw having a planar surface and an open ended channel formed insaid first jaw opening to said planar surface; and a second jawincluding an upper jaw mount and an upper jaw assembly comprising: asupport flange attached to said upper jaw mount; a fiber clasp having acontact surface opposite a structured surface including an open endedgroove of substantially rectangular cross section; and a plurality ofspring connectors to hold said fiber clasp to said support flange, saidchannel and said groove sized for even application of pressure to thefilament placed therebetween, when there is axial alignment of saidchannel with said groove, and said planar surface lies adjacent saidstructured surface, said filament holding fixture being in said closedposition due to application of force to cause said first jaw to movetowards said second jaw.
 10. The filament holding fixture of claim 9,wherein said support flange includes a support surface having asubstantially conical recessed portion formed therein and said fiberclasp includes a contact face having a substantially conical depressedportion formed therein.
 11. The filament holding fixture of claim 10,further including an angular compensator confined between said recessedportion and said depressed portion by force produced by said pluralityof spring connectors, said angular compensator maintaining separationbetween said support flange and said fiber clasp to allow independentmovement of one from the other.